Mac suddenly won't allow me send or receive SMS/MMS in Messages

I can no longer send or receive SMS messages using Messages on my Macs. I didn't do anything other than stay current with upgrades across the board, and I have spent literally hours going around in circles trying everything and anything to figure this out: In Settings>Messages on my phone I don't have "Text Message Forwarding" as an option, which is mentioned by everyone as where I need to go to fix this. I'm using 17.5.1 on a 12 mini.

It's hard to say how long it has been this way because I have only one (!) SMS (or whatever) person in my list of people I chat with and just fired up a conversation today for the first time in months. Everything works great from the phone of course but no longer from my Mac. Believe me I have tried everything. Just for context Ive been using macs and iPhone since they each came out. Not a brag but just to say I'm reasonably familiar with this all in general but this has me stumped! The attached image shows the relevant parts of the settings>messages window.

HELP? ...and thanks in advance anyone!