Content Error

I have an error with my website. The code reads

Fatal error: Call to undefined function similar_posts() in /home/mikezxrs/public_html/accessbritney.com/wp-content/themes/AccessBritney_2013/single.php on line 69

I recently changed the domain from .net to .com and started with a whole new WordPress. I’m not sure if that’s where the problem is coming from.

The actual page is http://accessbritney.com/2016/04/15/britneys-new-single-is-titled-make-me-oooh/