I also have the same problem. I updated via the iPad and as soon as I tried to sync, I could not. I am locked out of syncing my books, music and recordings. I cannot update readily because of my need for older software.
This is the type of mistake that takes away the joy of using apple products.
Help Anyone. Not clear on how to proceed.
I also backed up my data on the IPAD and tried to go to a windows machine to restore directly from the backup folder placed on an external HD. No directions to be found to solve even at this level.
An update needs to come out or else my investment of a macbook pro and an iPad workflow has disappeared.
I am extremely disappointed.
All of my data is trapped. I can't even go backwards to iOS 11.4 to solve the problem. That should atleast be fixed if you have a back up.