If you have not yet, a prerequisite for the GPO enrollment is Azure AD Hybrid Join. You can find directions on how to accomplish this here
You can also find some more background information on it here
Once you have that completed and are running the correct version of windows, I recommend at least 1803, and have your GPO store updated as such you can create the new GPO and deploy it to your Hybrid Joined Devices. Information on that process can be found here.
What this article from Microsoft doesn't tell you is where you can find the event logs for this process or what the error codes you might find are. The location in the event viewer is
Applications and Services Logs/Microsoft/Windows/DeviceManagement-Enterprise-Diagnostics-Provider/Admin
MS does offer additional tshooting help in some tucked away corners of their platform that I want to gather here. Use the below links as a starting point. Good luck!