I don't think you're doing anything wrong, it sounds like the built-in administrator account has been disabled by some means, I could only speculate as to how. Let's see if we can get it back.
Normally to force it to be active, you could type this at an elevated command prompt:
net user administrator /active:yes
However this requires that you are able to login to the computer first to open the command prompt and type that command.
So if you cannot do that, the next thing I would try if it were my PC is a Refresh or Reset operation. Unfortunately these are both destructive in some way. Refresh is the safest because it doesn't remove any document/picture files from your account, but it's also unlikely to resolve the issue. But the next step is more drastic, because Reset puts your PC back the way it came from the factory out of the box: literally everything is gone and it's reset like it was brand new.
+I to display the Settings charm menu.