"MMS Messaging needs to be enabled to send this message."

Just started getting this message in the last couple of days when I try sending texts and MMS using Messages on my MBP. Not having an issue sending from either my iPhone or iPad. And its just when sending to people who are NOT using Mac devices (The "Green Bubble" people). I've checked my phone and MBP. All settings are correct, as far as I see. "Text Message Forwarding" is set up correctly.

This has just started. Worked fine, and then it didn't. I did not change any settings. Help!