Why can't I send a message on my Xbox?

Are they your friend? I know some users have it so only friends can message them, so if they aren't your friend and they have it to where only friends can message them, the send message part will either be gone or you just can't send one. I'm not entirely sure on what it exactly does as I've never come across it. My friend told me that it just doesn't appear, but I don't know. It could be that, hope that helps.