Text messaging has stopped working on my iMac

I have an iMac running High Sierra 10.13.6 and an iphone 6 running ios 12.2. For the past several years i've been texting a group of friends using either my iMac or my iphone! Messages texted on one device would almost instaneously show up on the other. I encountered no real problems sending messages. Recently that changed. Now, for some reason, my iMac fails to deliver messages sent to my friends and the messages sent using my iphone no longer appear on my iMac.

When I send a message using my iMac i get the error message shown below.

If I send the message to the same person using my iphone the message is delivered without error.

Does anyone know why this is occuring? I've restarted the Messages app and rebooted my iMac. Still no luck.

After comments...

It's often good to just go through the old "switch it off then back on again" routine, no matter how trite it may seem ;)

In this case - sign out [from Prefs > iMessage] & then sign back in again.