WORDPRESS - include CSS in functions.php doesnt work

Question

I need to include some external css files, i use this snippet in functions.php but neither of them works.. Why..?

    add_action( 'wp_enqueue_scripts', 'add_css' );

function add_css() {
    wp_enqueue_style( 'parent-style', get_template_directory_uri().'/style.css' );
    wp_enqueue_style( 'style', get_template_directory_uri().'/assets/css/bootstrap.css' );
}

I'm using Twenty-Twenty-One child theme This is my style.css

/*Theme Name: Twenty Twenty-One Child
    Theme URI: https://wordpress.org/themes/twentytwentyone/
    Template: twentytwentyone
    Author: the WordPress team
    Author URI: https://wordpress.org/
    */
h2{
    color:red!important;
} */

h2 is to see if it works.

Answer 1

If you're using a child theme, you want to use get_stylesheet_directory_uri() instead of get_template_directory_uri - Unless the stylesheets are part of the parent theme.

function add_css() {
    wp_enqueue_style( 'parent-style', get_stylesheet_directory_uri().'/style.css' );
    wp_enqueue_style( 'style', get_stylesheet_directory_uri().'/assets/css/bootstrap.css' );
}