Error: Logging in to the account failed because an error occurred.

I reinstalled Big Sur and no help.

I found these instructions helpful:

1. Shut down your Mac.
2. Reboot your Mac and press and hold “Command + R“.
3. At the “macOS Recovery”-Screen select a user, click on “Next” and enter the user’s password. Note: If you do not know the password click on “Forgot all passwords” and sign in with your Apple ID. This step is needed if you secured your Mac with FileVault.
4. On the next screen select Disk Utility and mount all internal discs, especially “Macintosh HD – Data“.
5. Close Disk Utility and open Terminal via selecting Utilities and then Terminal.
6. At the prompt (in my case [-bash-3.2#]) type cd /Volumes/”Macintosh HD”/var/db and hit enter tab.
7. Run the following command to remove file “.AppleSetupDone”: Type rm -f ./.AppleSetUpdone and hit enter tab.
8. Then reboot your Mac: Type reboot and hit enter tab.
9. Your Mac reboots. Go through the Setup Process. Create a new account and take a DIFFERENTusername than your Standard user account (e.g. “Backup Admin”)
10. After logging in, open System Preferences and select your Standard user account and check the box labeled “Allow user to administer this computer” (Note: if the box is grayed out, click the lock icon the lower left to enabled editing)
11. Log out of your Mac and log in in as your original account.
12. Either delete the Admin account you created in step 4 or, even better, leave it as a backup account.

Hope this is helpful to others.

Update, am able to access Recovery Mode. Any suggestions on where to go from here?

Should I re-install Big Sur?

Or is there a command line that would allow me to create a dummy admin user that I can log into from login screen? I tried andyball_uk's command rm /var/db/.AppleSetupDone from a thread from years ago with Snow Leopard, but terminal says no such directory exists so I think things have changed from OS's.