iPhone backup
when i try to back up my iPhone on pc iTunes says cannot back up unknown error -54 occurred
MacBook Pro with Retina display, OS X El Capitan (10.11)
error 54 - I don't think it an Apple or ITunes error code as I don't see it on the list.
Get help with iOS update and restore errors - Apple Support
iTunes: Possible iTunes Store errors - Apple Support
Google error 54 and see what is yielded.