I spent most of Friday slowly restoring 14 iPads back to the state that they were in at 3pm on Thursday afternoon before I attempted a simple OS update via Apple Configurator 2; which bricked them all and lost me my nights sleep. It was also at this point that my (now deleted by Apple*) reply to this post, was published; in a state of fury.
The method which was successful for me was to use Finder to upgrade One device, to iPad OS 14 and then to upgrade via AC2. Of course then I had to Prepare the devices and also apply the Blueprint for the School. This is despite AC2 saying its downloading iPad OS, again, after you've already done it via Finder.
This has happened in 2 schools I support now, but the other error message was more helpful in the first instance, it said, The system restore image is invalid. Which helped me arrive at the correct solution.
My constructive advice to Apple would be to rewrite this from the ground up, and to understand that many schools are using Mac mini's or iMacs that are several years old. Support should be equal for the schools who are struggling with finances and the software should be compatible across all Mac's and OS. Also, teachers use this, so error messages that only make sense to a developer and don't address the actual issue only help confound the situation.
(* it was deleted for not being constructive enough).