Before going further, I would appreciate your help in clarifying the following situations:
Do you wish to turn Windows Defender off and use your AntiVirus?
You could try uninstalling your Antivirus, reboot the system, and reinstall.
Third-party Antivirus apps would usually take over once it is activated. If the app wouldn’t launch, you could contact the third-party support.
I hope this information above can help you.