Just updated to Big Sur, can't login anymore

Solution 1:

When you say you tried enabling the setup assistant, what specifically did you do?

In the past, booting to single user mode was simple but based on which MacBook you have and details you didn’t provide in the question (firmware status, FileVault status, DEP status, MDM status), you might be making things worse if you’re not following the Apple procedure precisely to recover a lost password on Secure Enclave Macs with always on encryption.

• https://support.apple.com/en-us/HT202860

I know the comments say you think the password is correct, but I suspect you’ve gotten the encryption in a disjoint state.

• https://support.apple.com/en-us/HT204156

If you are sure the password reset won’t help, try one time to reinstall the OS from recovery then look at data recovery options if you don’t have a backup or erase install if you don’t care to protect any data now on the Mac.

• https://support.apple.com/en-us/HT201314