I finally have it working but required a week and a half of lost time of several people, and trying all the standard FCPX solutions.
Had to get another iMac that was on Catalina and upgrade it to Big Sur, because apple doesn't make it easy to downgrade software that isn't working! Several techs said I'd have to wipe the machine clean and restore from a time machine backup. This is really criminal.
Got is working though -- we think. At least it is loading and opening. we will see if there are anymore problems. I was on High Sierra on this edit that has been going on since 2016. For independent filmmakers, whose projects last many years, FCPX is not a viable option if they can't make it easy to collaborate with other editors and professionals or to do basic post-production activities how the bulk of professionals out there do it. The only way we go it to work was to be on the exact same OS and version of FCPX, but to do this, is not straight forward.