I wanted to put parental controls on my computer before I knew that admin users can't have parental controls (now it makes sense to me). But somehow I managed to enable parental controls for my admin user, and it didn't give me any warning message.
I can tell that enabling parental controls really worked because some websites are blocked. This persists even after restarting my Mac.
Now I want to disable parental controls, but I can't do it. When I try to disable it in System Preferences, I get this error message (video):
You cannot enable parental controls for an administrator account. Create a new user account and then enable parental controls for the new account.
I am using a MacBook Air (13 inch, mid 2011) with macOS High Sierra 10.13.6.