How to add an admin user to sudoers on Big Sur [duplicate]

Question

I am running Big Sur on a MacBook Pro and am unable to run su or sudo in a terminal window despite being logged into an Admin (and Mobile) account. I always get the error: <username> is not in the sudoers file.

Here is a transcript:

mac-947:~ spertus$ visudo
visudo: /etc/sudoers: Permission denied
mac-947:~ spertus$ sudo visudo
Password:
spertus is not in the sudoers file.  This incident will be reported.
mac-947:~ spertus$ sudo su
Password:
spertus is not in the sudoers file.  This incident will be reported.
mac-947:~ spertus$ sudo -s su
Password:
spertus is not in the sudoers file.  This incident will be reported.
mac-947:~ spertus$ sudo -i spertus su
Password:
spertus is not in the sudoers file.  This incident will be reported.

As suggested in a comment to macOS - User can perform admin tasks but cannot sudo, I tried granting Full Disk Access to the terminal app, but that didn't help. I also tried the answers in Cannot su to Admin on Big Sur without success.

This shows that I am logged into an account in the Admin group:

How can I add my account to the sudoers file (or otherwise run with root privileges)?

My question differs from I don't have administrator account on my mac because my account is in the Admin group, as shown in the screenshot.

Answer 1

You can try create a new Admin User.

On Big Sur the following method works fine :

You boot in Recovery Mode. (cmd+r at boot) .

If Filevault is activated, from Disk Utility, you unlock/mount the System Volume giving the password. (right click on the system volume).

You quit disk utility, then on top menu you select Utilities/Terminal

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.

Then you may be could repair the currents Users.