Undefined Function Showing on iPad/iPhone but NOT on Desktop Display

Hoping someone can help me with an aggravating problem I’m having. When I view my site here: www.cincocharities.org in a desktop browser, everything looks fine and dandy, but when I view it on an iPad or iPhone, it returns the following error:

Fatal Error: Call to undefined function has_post_thumbnail();…

Extensive googling keeps telling me the add_theme_support( ‘post-thumbnails’ ); must be included in the functions file which it is (and even if it wasn’t, wouldnt it also error out on the desktop view).

Someone please help!!!