SQL Server principal "dbo" does not exist,

Question

I resolved this issue by setting database owner. My database did not have had any owner before this issue. Execute this command in your database to set owner to sysadmin account:

use [YourDatabaseName] EXEC sp_changedbowner 'sa'

Do Graphically.

Database right click-->properties-->files-->select database owner-->select [sa]-- ok

USE [<dbname>]
GO
sp_changedbowner '<user>' -- you can use 'sa' as a quick fix in databases with SQL authentication

KB913423 - You cannot run a statement or a module that includes the EXECUTE AS clause after you restore a database in SQL Server 2005

After restoring a Database from SQL2016 to SQL2019, I had the same issue when I try to access Database Diagrams. I had the correct Database owner already but owner of Files was empty. Once I set that, it worked properly...

This may also happen when the database is a restore from a different SQL server or instance. In that case, the security principal 'dbo' in the database is not the same as the security principal on the SQL server on which the db was restored. Don't ask me how I know this...

another way of doing it

ALTER AUTHORIZATION 
ON DATABASE::[DatabaseName]
TO [A Suitable Login];

Selected answer and some others are all good. I just want give a more SQL pure explanation. It comes to same solution that there is no (valid) database owner.

Database owner account dbo which is mentioned in error is always created with database. So it seems strange that it doesn't exist but you can check with two selects (or one but let's keep it simple).

SELECT [name],[sid] 
FROM [DB_NAME].[sys].[database_principals]
WHERE [name] = 'dbo'

which shows SID of dbo user in DB_NAME database and

SELECT [name],[sid] 
FROM [sys].[syslogins]

to show all logins (and their SIDs) for this SQL server instance. Notice it didn't write any db_name prefix, that's because every database has same information in that view.

So in case of error above there will not be login with SID that is assigned to database dbo user.

As explained above that usually happens when restoring database from another computer (where database and dbo user were created by different login). And you can fix it by changing ownership to existing login.