I had the same problems as everyone else, and I tried all the recommendations with no success. The I realized it was only one app that was causing the problem.
The iPad had belonged to my father who sadly passed away at 93 recently and I was trying to set the iPad up for my wife to use. Everything was fine except for the Facebook app which I thought had deleted. The App Store wanted to "update" it and kept asking for his Apple ID. However, there was no icon for the Facebook app on any screen.
Then remembered a similar issue with my wife's iPhone where the App Store icon had disappeared. Going to Settings > General > Reset > Reset Home Screen Layout fixed it. I did the same with my Dad's iPad and *bingo* there was the Facebook app. For whatever reason, it had become hidden. I deleted it, went back to the App Store and it let me download the Facebook app with my own Apple ID.
So if all the other recommendations in this discussion fail, try resetting the home screen layout. You might have a hidden app that needed to be deleted first.