Hey Carol Baker,
I see that you're having some trouble using FaceTime on your MacBook Air, though it works on your iPhone 8.
First, make sure that FaceTime is confused on your MacBook Air with the correct Apple ID: Use FaceTime on your Mac - Apple Support
If you're still having trouble after following the steps in the resource above, let us know at what point do you receive this error message that you noted in your response? Do you receive it when opening FaceTime or when initiating a phone call?
Take care.