Suddenly unable to send SMS from macOS (names marked RED)

Solution 1:

On your iPhone, do you have an email address checked under settings>Messages>"send & receive"?

for some reason, it only works when I have an email address checked which was not the case before. They must have changed something on their end to require the use of an email.

Solution 2:

Thank you! This worked for me. Had to had email. Went back to just phone number and text only contacts turned red again and would've send messages. Could receive though. Re-added email and Contacts turned green again. They must have changed something on their end.

Solution 3:

I had the same problem just now, and user233886 gave me the idea to try something on my iPhone. I went to Settings > Messages > Send & Receive, unchecked all my email addresses, waited a few seconds, then checked the email addresses again.

My Mac immediately asked if I wanted to allow something, and I accepted. After about 30 seconds, the names were no longer red in the Mac Messages app.