csrutil disable
Restart computer
In the terminal, type
sudo open /Applications/TextEdit.app /System/Library/LaunchDaemons/com.apple.ManagedClient.enroll.plist
change
<key>com.apple.ManagedClient.enroll</key>
<true/>
to
<key>com.apple.ManagedClient.enroll</key>
<false/>
So that the changes take effect
Yes! I bought one at the OLX sales market, I didn't know and when I formatted it I got this surprise. I solved it with this post. Oh! After solving don't format, just sign out of icloud, open a new user and delete the old one.