Undefined function related_posts() in single.php

Hi, I have some problems using YARPP plugin in my wordpress site, I have the thematic theme and the wordpress is multi user.
I need to add the related_posts function in my homepage and in each single post page. I added and activated the plugin in my WP and I added the related_posts function in the header.php.
For some reason in the homepage the plugin works fine but in others pages e.g post page (single.php) the site returns me undefined function related_posts().
I dont know what is the problem 🙁

Thanks!! your comments will be appreciated!

https://wordpress.org/plugins/yet-another-related-posts-plugin/