Fatal error

I wanted to move YAARP below comments, so I am using php related posts in single.php as: <?php related_posts(); ?>

While it looks perfectly fine, my error log is filling up with

PHP Fatal error: Call to undefined function related_posts() in //wp-content/themes/my-child-them/single.php on line 135

Any ideas?

thanks

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