Another only admin acct hidden

(Thanks to W10 Tweaker for the redirect.)
Ref: Hidden Administrator account, no backup admin

W10H 1909 (18363.815), ASUS laptop i5-8250, x64, 8GB, 250SSD.

So I did the same thing. Working on my son's computer, annoyed that everybody's login shows up on the front screen. He knows we have access to his computer, just don't need it staring at him in the face every day. (At work after restart only the latest user shows up... presume that's b'c it's an enterprise Windows version.) So for his I used the hide admin acct in the registry trick to get it off the login screen. Now I can't log in! (Figured it would just give me an "other user" option. Dumb.)

So that's the only admin acct on his computer. So I did a restart holding shift and did the whole Troubleshoot>Advanced Options>Command Prompt and then ran it as administrator (net user administrator /active:yes)

I used Bree's trick in this thread and got to step 7 of Option 4 and get the following error:
Cannot Load X:\Windows\System32\config\SAM: The process cannot access the file because it is being used by another process.

I tried restarting to see if I was missing something that I duplicated. And I can't get in to run CMD as admin under other accts b'c it doesn't give me a PW option, only fingerprint which was never set up or "No."

Just want to be able to log into the admin acct and/or remove the hidden so I can.

Thanks,
Duh in Florida