FaceTime has not received any video from the connected camera. Restarting your computer may fix this issue. I get this error message on FaceTime when I try to use it.

Hello joriep1,

Thanks for that info and for choosing the Apple Support Communities. If we understand correctly, you are having an issue with FaceTime on your Mac. If you haven't already, please follow these steps and test your issue again:

"If your camera or microphone doesn’t work

• Quit FaceTime, then open FaceTime again. 
• Restart your Mac.
• Make sure that FaceTime isn't open, then see if your camera works in another app, such as Photo Booth. If it doesn't, you know it's not an issue with FaceTime.
• If you have multiple cameras or microphones connected to your Mac, check the Video menu in the FaceTime menu bar to make sure that you have the correct camera and microphone selected.
• If the issue involves your computer's built-in camera, learn how to resolve issues with the built-in camera.
• If the issue involves iPhone when used as a webcam, consult Continuity Camera: Use your iPhone as a webcam for Mac."

And those steps are from If FaceTime isn't working on your Mac - Apple Support

Cheers!