Symptom: No 3rd party antivirus installed, still cannot enable Windows Defender

run the following code in the command window:
sc start WinDefend Enable
then restart your computer.

Symptom: Popup message box says “This app is turned off by group policy”

The Group Policy is it referring to is probably the value DisableAntiSpyware under HKLM\Software\Policies\Microsoft\Windows Defender in the registry.
Just delete that value and Windows Defender should work again. (source)