I also encountered this error message for the first time today (currently July 2021). The message also states that the Mac cannot be used until the critical error is fixed but I was thrown into a continual error message loop and it would not update. I was running Big Sur 11.4 on a 15” MBP Touchbar Late 2016 and I was attempting to update the OS to 11.5. I read the other responses here, some of which I found to be helpful. What ended up working for me was starting in safe mode (hold left key “shift” during startup). While in safe mode I backed up my Mac through Time Machine to an external hard disk. I had not created a backup to this point (very bad on my part) and wanted to do so in case I could no longer access my data due to the critical error message. The backup took several hours as expected but completed without any problems. While still in safe mode, I went into System Preferences > Software Update and attempted to run the update to Big Sur 11.5 again. This time it worked. It updated correctly and restarted my Mac. Now I’m able to log into my machine without any issues. I hope someone finds this helpful as this was very discouraging because a lot of posts that I read about this error message ended with wiping the drive, a costly fix, or getting a new Mac! 😳