problem with comments using cherry berry theme

I am using the “Cherry Berry” theme downloaded from the WordPress site and keep getting the following error message when clicking through to “comments” link in my individual posts:

Fatal error: Call to undefined function: related_posts() in /home/momsaliv/public_html/wp-content/themes/cherryberry/single.php on line 20

No comment form appears, just this message.

Does anyone have a similar problem or know what I can do to fix this? Thank you in advance for your help.