Still seeing some desperate users here, so I will summarize some previous posts. Clear parameter ram: Reboot with Command-Option-P-R held down (fast, easy, if this works, great!) Reboot into safe mode: Hold down shift before the logo/load screen comes up. Reboot into recovery mode: Reboot holding down Command-Option-R In recovery mode, you can connect to wifi and do a network install. Takes a while, but one way to get a remote re-install if your computer won't boot. From Safe Mode, you can: a) Reinstall Big Sur b) Run Disk First Aid c) Backup your files If you reinstall, and come back to the same problem: A) backup your files, then do a Clean Install (this means you lose your files, so hence backup first) B) you can try to find the needle in the haystack (what was Big Sur trying to install from your previous system that is incompatible?) and remove it, but that requires some deeper expertise. These are all Mac101 troubleshooting tips. I'm currently having this problem, and this is what I'm doing to try and fix.