Hello, I hope that you are well. My I Message app will send messages only from my phone.
When I try to send any I message from my iMacs or MacBook Air they fail to send. My messages are not delivered from my iMacs or MacBook Air.
MacBook Air 13″
On your Mac, did you Sign In to iMessage using your Apple ID?
Use Messages on your Mac - Apple Support
If you are unable to do that, please read If you can't sign in to your iMessage account or FaceTime on your Mac - Apple Support.
If applicable: Forward SMS/MMS text messages from your iPhone to your Mac or iPad - Apple Support