I can't send iMessages from my Mac?

Recently I have not been able to send or receive any imessages from my Mac. I can send/receive them from my iPad mini.

When I try to send a message their name turns red and it says that "insert email/phone" is not registered with iMessage.

Like I said, this problem is recent, and I have been able to send them before, so I am not sure what it could be. I have tried

logging out of iCloud on my Mac, and Logging out of iMessages as well. It hasn't helped yet.