Missing admin account in Big Sur, pre-Big Sur solutions do not work
Solution 1:
If you need create a Admin User on Big Sur the following method works fine :
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You boot in Recovery Mode. (cmd+r at boot) .
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If Filevault is activated, from Disk Utility, you unlock/mount the System Volume giving the password. (right click on the system volume).
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You quit disk utility, then on top menu you select Utilities/Terminal
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I suppose the System Volume name is "Macintosh HD", the command is :
rm /Volumes/"Macintosh HD"/var/db/.AppleSetupDone -
If no error message you reboot the Mac and then you will be invited to create a new administrator.