Sending a text message from my Mac doesn't work

Have you enabled Text Message Forwarding for your Mac? On your iPhone, go to Settings>Text Message Forwarding and add your Mac.

See also:

Forward SMS/MMS text messages from your iPhone to other devices - Apple Support

bigbaacke wrote:

Yes. Text message forwarding is set on my iPhone. I don't want to forward a message to my iMac, I want to send a new message from my iMac.

Your Mac is not capable of sending SMS messages by itself. It must be properly connected to the iPhone. And it seems as if it is not.

Try removing the Mac from Settings>Messages>Text Message Forwarding on your iPhone. Then, turn off iMessage on your Mac. Restart it. Re-enable iMessage. Once it's activated, go back to the phone and re-enable Text Message Forwarding.

bigbaacke wrote:

The messages I send with my iPad or iPhone appear on my iMac, but I can't send a message from my iMac.

Did you follow the instructions I gave above? Did you read the article I linked to?

bigbaacke wrote:

That got the job done!!! Thank you so much for your help!

Glad to hear it!

Sometimes things get a bit gummed up and logging out and logging back in clears it.