Same problem. Error 56.
I have an iPad Pro 9.7 inch 256 GB with cellular.
My iPad worked fine before the update.
Trying to repeat the update multiple times did nothing except generate more error messages.
So, I bit the bullet and selected the restore option. This resulted in a warning that all of my data and settings will be deleted. I had spent many dozens of hours getting everything just right on my iPad before this update problem. I had downloaded many GBs of hiking maps, music, magazines, books, etc. and it is all lost. I wonder why the update, which was created to fix a problem that I did not have, was not thoroughly tested before it was released.
I use my iPad every day to check traffic on my way to work, etc. Without a functioning iPad, I am in a bad situation.
The odd thing is, despite three error messages saying that even restore has failed, I noticed that the update progress arrow in the upper right hand corner in iTunes on my iMac shows that the update is still downloading. It shows 48 more minutes to go to download just 1.1 GB remaining. Now I am ignoring the multiple error messages and allowing the update to download.
I have work to do from home tonight. I don't have time for these problems.
I checked three tech sites and none of them had posts about problems updating.
I have owned every iPad since it was first introduced.