You can create a new administrator account by restarting the Setup Assistant

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Boot into Single User Mode: Start/restart your Mac. As soon as you hear the startup tone, press and hold ⌘ + S until you see a black screen with white lettering. (If you end up back on the login screen after a flash of the black screen with white lettering, enter your password and it will return to the black screen.)
Check and repair the drive by typing /sbin/fsck -fy then ↩ enter - as directed by the on-screen text.
Mount the drive as read-write by typing /sbin/mount -uw / then ↩ enter.
Remove the Apple Setup Done file by typing rm /var/db/.AppleSetupDone then ↩ enter.
Reboot by typing reboot then ↩ enter.
Complete the setup process, creating a new admin account.

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