FaceTime: unable to answer incoming calls on macOS
Try rebooting in safe mode as shown in this apple guide: https://support.apple.com/en-us/HT201262 and then restart your mac normally.
This should clear the caches and fix the issue.

Source: https://medium.com/@ambroselittle/fix-call-failed-with-continuity-on-macos-sierra-e9e662ad6947