Custom post type editor uses old tinyMCE

In your code where you defined your custom post type (in a custom plugin or in your functions.phpfile), you need to add this snippet for Gutenberg block editor support:

'show_in_rest' => true,
   'supports' => array('editor')

Here is an example:

function portfolio_post_type() {
    register_post_type( 'portfolio',
        array(
            'labels' => array(
                'name' => __( 'Portfolio' ),
                'singular_name' => __( 'Portfolio' )
            ),
            'has_archive' => true,
            'public' => true,
            'rewrite' => array('slug' => 'portfolio'),
            'show_in_rest' => true,
            'supports' => array('editor')
        )
    );
}
 
add_action( 'init', 'portfolio_post_type' );