Hi there ,
Welcome to Apple Support communities. We understand that you're unable to activate and use FaceTime on your Mac. You mentioned working through a few steps, but if you haven't already make sure your macOS is up-to-date in Apple menu () > About This Mac > Overview, check that your time and date displays accurately, and check to make sure you can sign in with your Apple ID on the Apple Website: Apple - Manage Apple ID
Try connecting to a different network and signing in to FaceTime. If you installed firewall, security, VPN, or other third-party networking software, make sure that the software isn't blocking any of the ports that iMessage or FaceTime needs. If necessary, disable the software and then try using your iMessage account or FaceTime again. If you use FaceTime and iMessage behind a firewall provides more insight on port numbers used.
Regards.