Fatal error undefined function

I want to move YARRP below comments, so I set YARRP not to show and then added

<?php related_posts(); ?>

in single.php

PHP Fatal error: Call to undefined function related_posts() in /my-theme-path/single.php on line 133

Am I missing something?

Thanks

https://www.ads-software.com/plugins/yet-another-related-posts-plugin/