"messages can't start the chat"

I'm getting the message Messages can't start the chat, xxxx # is not registered with iMessage.

Here's the thing...

I really wasn't expecting this to work on my old macbook running Mountain Lion (works flawlessly on my mini running Yosemite). I thought it wasn't supported at the Mtn Lion level. Regardless, today I was chatting with both of my sons…. me on my macbook, them on their iPhones. **HOWEVER, I am unable to initiate a chat with my wife from the macbook. Phone to phone works fine, both directions, but doesn't appear in my Messages window on the macbook.

Any thoughts?

T.