Just happened to me with Big Sur (2/2/21) - when i uninstalled Norton internet security (because Xfinity decided not to provide it for free anymore) and i had to reboot after the uninstall process. I started back up in safe mode (like another person suggested) and it came back up to let me install updates, which so far it is successful in doing. If anything goes wonky, I'll update.
mellooney
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