Messaging works on iPad, not on Mac

I have NOT upgraded my iPad 2 to 8.xxx, due to the warnings on these boards, and other Mac sites. My Mac is running Yosemite 10.10.3. Is it possible that messaging isn't working on my desktop Mac because I haven't upgraded my iPad to 8.xxx?

Last week, iMessage stopped working on my Mac. It says I am signed in, but I just get this error over and over, and a red-colored not-delivered message on the text.

I have done at least a dozen restarts, have signed in and out of my AppleID, restarted the modem... Messages Preferences says I'm logged in. But I have noticed that Messages is not even listed as an app in the iCloud preferences window.