(A) Add a user, restart in Safe Mode and see if you can add the user to the Admin group.
You can enable the normally disabled Administrator to do this.
Also try these steps:
Quick guide: Enable administrator account in Windows 10
Open “Run” with [Windows] + [R].
Type “cmd” and press [Ctrl] + [Shift] + [Enter].
Type “net user administrator /active:yes”.
The administrator account is now activated.
To deactivate: “net user administrator /active:no”.
You need to login as Administrator when done
Enable built in Admin Account
Enable: Search for cmd in the taskbar search field > select Run as Administrator.
Type net user administrator /active:yes, and press enter. Wait for the confirmation and restart.
To disable, open the command prompt as an administrator and enter net user administrator /active:no
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(B)
If all the above fails, then back up the machine and reinstall Windows fresh.