Why does my iPhone say iMessage needs to be enabled?

Pop up says "iMessage needs to be enabled to send this message"

I messages is on on my iPhone. When I try to send a message I get a pop up that says, "iMessage needs to be enabled to send this message". I've gone to settings and turned it off and on several times and rebooted the phone several times. I still get the message. Please help

[Re-Titled by Moderator]