Hello marcmeyer,
Thanks for participating in Apple Support Communities!
We understand you are running in to an issue where you are unable to initiate a FaceTime call for one specific contact on your Mac. We are happy to help go over this with you.
There might be something stored within that contacts details that could prevent it from being able to have FaceTime initiated. Do you know if the email or Apple ID you have saved for that contact is the same email or Apple ID they use for FaceTime? Are there multiple email addresses saved for that contact?
Is your macOS version up to date? Take a look at these steps for how to update your macOS:
How to update the software on your Mac
Have you tried removing and re-adding the contact on your Mac? Give that a try and then try initiating another call:
Delete contacts or contact groups in Contacts on Mac
We hope this helps!
Best Regards.