To make use of meta data in a Custom Post Type remember that custom-fields needs to be set in the supports property of the register_post_type function.
To activate the Gutenberg/Block editor in a Custom Post Type editor needs to be set in the supports property. show_in_rest also needs to be set to true.
register_post_type( 'my_cpt', array(
'show_in_rest' => true,
'supports' => array(
'custom-fields',
'editor',
),
// ...
);
PHP