In WordPress, if you want to add support parameter in custom post type, here is solutions. Here i was create “music reviews” custom post type. In this custom post type i was add new “test example” post. In this post you can see “excerpt” parameter add in this post.
For this, Add below line of code in your theme’s functions.php file of your active child theme (or theme) and save the file.
add_action('init', 'support_parameter_add_excerpt', 100);
add_post_type_support('music_review', 'excerpt'); // Instead of "music_review" add your page / custom post slug here