IPhone message send failure?

My phone won't let me send a message. I have service and wifi and it lets me send messages to other people just not to the person I'm trying to send it to. I've tried restarting my phone but every time I try to send it, it fails right away.

Maybe that person's iphone is off or something happened to it. If that's the case, send the message and tap on the message you sent for 1 second and then the option that says "Send as Text Message" should appear, click on that.