LOL and I thought I was the only one.
This “Your Purchase Could Not Be Completed. For assistance, contact iTunes Support at at www.apple.com/support/itunes/ww/.” error is frustrating!
I tried everything and it still shows this stupid error. :(
EDIT. I have managed to find my issue it was something related to Unrestricted Access where in my iOS settings there's this Restrictions setting that was enabled. After that i proceed to sign in and out of my account and it worked!! This is the guide that helped: How to Fix "Your Purchase Could Not Be Completed" Error - Saint
Hope this helps you. Still Apple has to do something about it.