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″
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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