On startup, click on "Other"

Enter username: root and leave the password empty. Press enter. (Try twice)

If you're able to log in (hurray, you're the admin now), then head over to
System Preferences>Users & Groups and create a new Admin

Now restart and login to the new Admin Account (you may
need a new Apple Id). Once you're logged into this new Admin Id, you can
again proceed to your System Preferences>Users & Groups. Open
the Lock Icon with your new Admin ID/Password. Assign "Allow user to
administer this computer" to your original Apple ID. Restart.