Can't unlock admin users account
I was cleaning an old iMac (27" 2011 - OS 10.13.6) to give to nephew. I followed instructions to change user name and password. Hit 'restart' then realized I wanted a screen shot of the info as a reference for the new owner. Stopped the shut down. When I opened the Users & Groups (advanced) window again, it was mostly blank now. Continued the shut down (big mistake).
Now I can see the User Name, I can log in to the account (so the password works) but when I click the lock to allow changes, the user name does not work - I assume the password does as it does allow me to log into the admin account.
I have tried resetting the password again through the terminal, and yes I can change the password.
I have listed the users in the terminal (Ls/Users) and I can see the name, but when I put the name into 'user name' it doesn't work.
Yes I'm a dummy, but how does this dummy fix this?
-rob