I am having the same issues as described here. I have 3 machines, all with Beta 6 installed: Mac mini (2018) iMac 27" (2020) MacBook Pro 13" (2020) Each of them will attempt to install either Beta 7 or Beta 8 from Software Update, and then fail during the reboot - stuck with the progress bar at ~80%. If I then switch off and restart, it will fail at the same point. However, I CAN go in to Safe Boot and it will work - not sure why. But then, if I go to Recovery Mode and reinstall, it goes back to Beta 6. Any help gratefully received!

Now trying to update to Beta 9 (released today) and the update is stuck at exactly the same point in the process.

The inability to upgrade from beta 6 to later betas is now logged under FB8756525.