Fatal Error with Grid Market theme and Marketpress Plugin

I am still looking for someone to help me solve a fatal error I am getting when I try to access an online store for a website I'm creating. I've submitted another post a while ago and nobody was able to help me. Can someone please step up?

I am using Marketpress Lite Version 2.5.1 I have activated the child theme for gridmarket. When I go to access any of the store I get this error:

"Fatal error: Call to undefined function mp_products_filter() in /home/content/18/10790718/html/wp-content/themes/gridmarket/mp_category.php on line 8"