Custom Hooks

I used the following code, but it is not displaying with or without the PHP part. Think I need to add something else? I have a theme that lets me add this to one post type, but not another. My goal is to use this on the Free Posts, but not the Membership Related Posts. So, I need Shortcode to tell it when to display. Thanks.

<?php related_posts(); ?>

https://ww.wp.xz.cn/plugins/yet-another-related-posts-plugin/