I have this problem on my new iPad as well on iOS 15 and it's driving me CRAZY!!! None of the stock solutions like restarting, charging overnight work! HELP please....

I have the same problem. Tried to upgrade my 2017 Macbook Air with Mojave to Big Sur 11.6 yesterday and after the whole day had to give up and use Time Machine to go back to Mojave. Tried safe mode, Command R, rebooting....nothing worked...I could boot into safe mode and it was Big Sur installed but it is not useable and it could never boot normally.