The brackets shown in the NET USER syntax are documentation notation. You do not type them. Also, “Administrator” is not something you put into the command unless that is actually the username you want to create.
From an elevated Administrator Command Prompt, use:
net user NewAdmin MyPassword123! /add
For example:
net user BackupAdmin MyPassword123! /add
You should then see “The command completed successfully.”
Next, add that account to the local Administrators group:
net localgroup Administrators BackupAdmin /add
Again, you should get “The command completed successfully.”
You can verify the account with:
net user BackupAdmin
And verify its group membership with:
net localgroup Administrators
If you are currently at the Windows Recovery Environment Command Prompt rather than a normal elevated Command Prompt, NET USER may be operating against the recovery environment rather than your installed Windows system. If the commands appear to work but the new account does not appear on the Windows login screen after rebooting, that is likely the issue.
If the above response helps answer your question, remember to "Accept Answer" so that others in the community facing similar issues can easily find the solution. Your contribution is highly appreciated.
hth
Marcin