Heroku deploy fails after upgrading to Ruby 2.6.5

Question

I migrated my app to Ruby 2.6.5 and Rails 5.2.3 recently. All the tests passed and the app acts normally on my local machine. When deploying to Heroku, I get the following error:

remote: -----> Detecting rails configuration        
remote: -----> Detecting rake tasks        
remote: [1m[31m        
remote:  !        
remote:  !     Could not detect rake tasks        
remote:  !     ensure you can run `$ bundle exec rake -P` against your app        
remote:  !     and using the production group of your Gemfile.        
remote:  !     rake aborted!        
remote:  !     LoadError: cannot load such file -- byebug        
remote:  !     /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/polyglot-0.3.5/lib/polyglot.rb:65:in `require'        
remote:  !     /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/polyglot-0.3.5/lib/polyglot.rb:65:in `require'        
remote:  !     /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/activesupport-5.2.3/lib/active_support/dependencies.rb:291:in `block in require'        
remote:  !     /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/activesupport-5.2.3/lib/active_support/dependencies.rb:257:in `load_dependency'        
remote:  !     /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/activesupport-5.2.3/lib/active_support/dependencies.rb:291:in `require'        
remote:  !     /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/is-release_pack-1.0.27/lib/tasks/is/release_pack_tasks.rake:2:in `<top (required)>'        
remote:  !     /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/activesupport-5.2.3/lib/active_support/dependencies.rb:285:in `load'        
remote:  !     /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/activesupport-5.2.3/lib/active_support/dependencies.rb:285:in `block in load'        
remote:  !     /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/activesupport-5.2.3/lib/active_support/dependencies.rb:257:in `load_dependency'        
remote:  !     /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/activesupport-5.2.3/lib/active_support/dependencies.rb:285:in `load'        
remote:  !     /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/railties-5.2.3/lib/rails/engine.rb:650:in `block in run_tasks_blocks'        
remote:  !     /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/railties-5.2.3/lib/rails/engine.rb:650:in `each'        
remote:  !     /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/railties-5.2.3/lib/rails/engine.rb:650:in `run_tasks_blocks'        
remote:  !     /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/railties-5.2.3/lib/rails/application.rb:514:in `block in run_tasks_blocks'        
remote:  !     /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/railties-5.2.3/lib/rails/engine/railties.rb:15:in `each'        
remote:  !     /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/railties-5.2.3/lib/rails/engine/railties.rb:15:in `each'        
remote:  !     /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/railties-5.2.3/lib/rails/application.rb:514:in `run_tasks_blocks'        
remote:  !     /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/railties-5.2.3/lib/rails/engine.rb:459:in `load_tasks'        
remote:  !     /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/railties-5.2.3/lib/rails/railtie.rb:190:in `public_send'        
remote:  !     /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/railties-5.2.3/lib/rails/railtie.rb:190:in `method_missing'        
remote:  !     /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/Rakefile:7:in `<top (required)>'        
remote:  !     /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/rake-13.0.1/lib/rake/rake_module.rb:29:in `load'        
remote:  !     /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/rake-13.0.1/lib/rake/rake_module.rb:29:in `load_rakefile'        
remote:  !     /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/rake-13.0.1/lib/rake/application.rb:703:in `raw_load_rakefile'        
remote:  !     /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/rake-13.0.1/lib/rake/application.rb:104:in `block in load_rakefile'        
remote:  !     /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/rake-13.0.1/lib/rake/application.rb:186:in `standard_exception_handling'        
remote:  !     /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/rake-13.0.1/lib/rake/application.rb:103:in `load_rakefile'        
remote:  !     /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/rake-13.0.1/lib/rake/application.rb:82:in `block in run'        
remote:  !     /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/rake-13.0.1/lib/rake/application.rb:186:in `standard_exception_handling'        
remote:  !     /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/rake-13.0.1/lib/rake/application.rb:80:in `run'        
remote:  !     /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/rake-13.0.1/exe/rake:27:in `<top (required)>'        
remote:  !     vendor/bundle/bin/rake:29:in `load'        
remote:  !     vendor/bundle/bin/rake:29:in `<main>'        
remote:  ![0m        
remote: /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/helpers/rake_runner.rb:106:in `load_rake_tasks!': Could not detect rake tasks (LanguagePack::Helpers::RakeRunner::CannotLoadRakefileError)        
remote: ensure you can run `$ bundle exec rake -P` against your app        
remote: and using the production group of your Gemfile.        
remote: rake aborted!        
remote: LoadError: cannot load such file -- byebug        
remote: /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/polyglot-0.3.5/lib/polyglot.rb:65:in `require'        
remote: /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/polyglot-0.3.5/lib/polyglot.rb:65:in `require'        
remote: /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/activesupport-5.2.3/lib/active_support/dependencies.rb:291:in `block in require'        
remote: /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/activesupport-5.2.3/lib/active_support/dependencies.rb:257:in `load_dependency'        
remote: /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/activesupport-5.2.3/lib/active_support/dependencies.rb:291:in `require'        
remote: /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/is-release_pack-1.0.27/lib/tasks/is/release_pack_tasks.rake:2:in `<top (required)>'        
remote: /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/activesupport-5.2.3/lib/active_support/dependencies.rb:285:in `load'        
remote: /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/activesupport-5.2.3/lib/active_support/dependencies.rb:285:in `block in load'        
remote: /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/activesupport-5.2.3/lib/active_support/dependencies.rb:257:in `load_dependency'        
remote: /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/activesupport-5.2.3/lib/active_support/dependencies.rb:285:in `load'        
remote: /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/railties-5.2.3/lib/rails/engine.rb:650:in `block in run_tasks_blocks'        
remote: /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/railties-5.2.3/lib/rails/engine.rb:650:in `each'        
remote: /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/railties-5.2.3/lib/rails/engine.rb:650:in `run_tasks_blocks'        
remote: /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/railties-5.2.3/lib/rails/application.rb:514:in `block in run_tasks_blocks'        
remote: /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/railties-5.2.3/lib/rails/engine/railties.rb:15:in `each'        
remote: /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/railties-5.2.3/lib/rails/engine/railties.rb:15:in `each'        
remote: /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/railties-5.2.3/lib/rails/application.rb:514:in `run_tasks_blocks'        
remote: /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/railties-5.2.3/lib/rails/engine.rb:459:in `load_tasks'        
remote: /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/railties-5.2.3/lib/rails/railtie.rb:190:in `public_send'        
remote: /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/railties-5.2.3/lib/rails/railtie.rb:190:in `method_missing'        
remote: /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/Rakefile:7:in `<top (required)>'        
remote: /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/rake-13.0.1/lib/rake/rake_module.rb:29:in `load'        
remote: /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/rake-13.0.1/lib/rake/rake_module.rb:29:in `load_rakefile'        
remote: /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/rake-13.0.1/lib/rake/application.rb:703:in `raw_load_rakefile'        
remote: /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/rake-13.0.1/lib/rake/application.rb:104:in `block in load_rakefile'        
remote: /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/rake-13.0.1/lib/rake/application.rb:186:in `standard_exception_handling'        
remote: /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/rake-13.0.1/lib/rake/application.rb:103:in `load_rakefile'        
remote: /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/rake-13.0.1/lib/rake/application.rb:82:in `block in run'        
remote: /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/rake-13.0.1/lib/rake/application.rb:186:in `standard_exception_handling'        
remote: /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/rake-13.0.1/lib/rake/application.rb:80:in `run'        
remote: /tmp/build_6756fdcd8f59060ad0f641b5f38a6571/vendor/bundle/ruby/2.6.0/gems/rake-13.0.1/exe/rake:27:in `<top (required)>'        
remote: vendor/bundle/bin/rake:29:in `load'        
remote: vendor/bundle/bin/rake:29:in `<main>'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/ruby.rb:961:in `rake'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/rails2.rb:79:in `has_jobs_work_task?'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/rails2.rb:53:in `block in default_process_types'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/instrument.rb:18:in `block (2 levels) in instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/instrument.rb:40:in `yield_with_block_depth'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/instrument.rb:17:in `block in instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/vendor/ruby/heroku-16/lib/ruby/2.5.0/benchmark.rb:308:in `realtime'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/instrument.rb:16:in `instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/base.rb:48:in `instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/base.rb:44:in `instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/rails2.rb:46:in `default_process_types'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/rails3.rb:29:in `block in default_process_types'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/instrument.rb:18:in `block (2 levels) in instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/instrument.rb:40:in `yield_with_block_depth'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/instrument.rb:17:in `block in instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/vendor/ruby/heroku-16/lib/ruby/2.5.0/benchmark.rb:308:in `realtime'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/instrument.rb:16:in `instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/base.rb:48:in `instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/base.rb:44:in `instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/rails3.rb:23:in `default_process_types'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/rails4.rb:26:in `block in default_process_types'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/instrument.rb:18:in `block (2 levels) in instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/instrument.rb:40:in `yield_with_block_depth'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/instrument.rb:17:in `block in instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/vendor/ruby/heroku-16/lib/ruby/2.5.0/benchmark.rb:308:in `realtime'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/instrument.rb:16:in `instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/base.rb:48:in `instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/base.rb:44:in `instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/rails4.rb:25:in `default_process_types'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/base.rb:105:in `write_release_yaml'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/base.rb:82:in `compile'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/ruby.rb:113:in `block in compile'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/instrument.rb:18:in `block (2 levels) in instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/instrument.rb:40:in `yield_with_block_depth'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/instrument.rb:17:in `block in instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/vendor/ruby/heroku-16/lib/ruby/2.5.0/benchmark.rb:308:in `realtime'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/instrument.rb:16:in `instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/base.rb:48:in `instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/base.rb:44:in `instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/ruby.rb:90:in `compile'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/rails2.rb:62:in `block in compile'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/instrument.rb:18:in `block (2 levels) in instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/instrument.rb:40:in `yield_with_block_depth'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/instrument.rb:17:in `block in instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/vendor/ruby/heroku-16/lib/ruby/2.5.0/benchmark.rb:308:in `realtime'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/instrument.rb:16:in `instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/base.rb:48:in `instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/base.rb:44:in `instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/rails2.rb:60:in `compile'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/rails3.rb:42:in `block in compile'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/instrument.rb:18:in `block (2 levels) in instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/instrument.rb:40:in `yield_with_block_depth'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/instrument.rb:17:in `block in instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/vendor/ruby/heroku-16/lib/ruby/2.5.0/benchmark.rb:308:in `realtime'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/instrument.rb:16:in `instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/base.rb:48:in `instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/base.rb:44:in `instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/rails3.rb:41:in `compile'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/rails4.rb:41:in `block in compile'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/instrument.rb:18:in `block (2 levels) in instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/instrument.rb:40:in `yield_with_block_depth'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/instrument.rb:17:in `block in instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/vendor/ruby/heroku-16/lib/ruby/2.5.0/benchmark.rb:308:in `realtime'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/instrument.rb:16:in `instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/base.rb:48:in `instrument'        
remote:     from /app/tmp/buildpacks/b7af5642714be4eddaa5f35e2b4c36176b839b4abcd9bfe57ee71c358d71152b4fd2cf925c5b6e6816adee359c4f0f966b663a7f8649b0729509d510091abc07/lib/language_pack/base.rb:44:in `instrument'        
...

Some other things that seem odd

What also confuses me is that I am using ruby 2.6.5, but in the log, the gems seem to be using ruby 2.6.0? Also, byebug is specified in the Gemfile as a development and test gem. Why is it installed on Heroku?

I find the line remote: Running: bundle install --without development:test --path vendor/bundle --binstubs vendor/bundle/bin -j4 --deployment strange, because --without accepts a space-separated list and here I can see a :-separated list.

More info

Except for these odd things, it seems that Heroku has successfully detected that the new Ruby should be used

remote: -----> Using Ruby version: ruby-2.6.5        
remote: -----> Installing dependencies using bundler 2.0.2        
remote:        Ruby version change detected. Clearing bundler cache.        
remote:        Old: ruby 2.4.6p354 (2019-04-01 revision 67394) [x86_64-linux]        
remote:        New: ruby 2.6.5p114 (2019-10-01 revision 67812) [x86_64-linux]        
remote:        Running: bundle install --without development:test --path vendor/bundle --binstubs vendor/bundle/bin -j4 --deployment

Answer 1

Also, byebug is specified in the Gemfile as a development and test gem. Why is it installed on Heroku?

Actually it isn't, and that's the reason of your error. The gem is missing, and somewhere in your code it seem to be required.

Especially look at the Rakefile file at line 7.

What also confuses me is that I am using ruby 2.6.5, but in the log, the gems seem to be using ruby 2.6.0?

You are using Ruby 2.6.x series so the folder is named 2.6.0 and the same would be if you'll use Ruby 2.6.0, 2.6.1, 2.6.2, you named it.

Hopefully this will help you.