I received a "Failed to prepare the software update. Please try again." message while trying to update from Big Sur 11.8.5 on an M1 mini. If would run for 10 minutes then stop with that message. I reloaded the software to try a total reinstall, but I also saw an error message on my system preferences. When I went to system preferences I saw a message that I needed to provide my iPhone 6 digit passcode to allow unified security. Once I did that the update ran all the way. Maybe related or may be a possible solution.