Thank you so much. This worked perfectly. I was about to give up, then entered "backup iPhone 6s plus using iTunes 12.8.2.3" on Google and found this post as the third entry. For anyone else out there, here is the info on my devices:
- iPhone 6s plus with iOS 13.3.1,
- connecting to a MacBook Pro (Retina, 15-inch, Mid 2014) using USB running OSX version 10.13.6 (High Sierra),
- using iTunes 12.8.2.3.
At first I was nervous to run the MobileDeviceUpdater.app, as your instructions said "it should notify you of a software update. Install it...". It was unclear what would be updated - the MacBook Pro OSX, the iPhone iOS, or iTunes update from 12.8.2.3 to something(?) else. From what happened, it appears that the update was only for a driver in iTunes and that the iTunes version stayed at 12.8.2.3.
Again, thanks so much!