If you previously had a 3rd party anti-virus installed, that will cause the problem and after being uninstalled that may have left some entries in the registry.
Click your Start Button, type regedit and hit Enter to open the Registry Editor.
Click View and make sure 'Address Bar' is turned on.
Paste this into the Address Bar at the top and hit Enter.
HKLM\SOFTWARE\Policies\Microsoft\Windows Defender
Please provide a screenshot of that Registry Editor page.