Messages problem: Unable to send to a person in my Contacts

MacAir with Monterrey OS 12.2.1.

Can send and receive messages to & from a person on all my devices except the MacAir with Monterrey. Keep getting the "not delivered" message. We can exchange texts on my iPhone, other devices not updated to Monterrey. I've reinstalled the OS but the problem persists. Contacts and messages are appropriately checked off in system preferences in iCloud. Frustrating as he is part of a group with which I need to stay in contact.