1. The other user only allows "friends" to send them messages.
2. You're trying to send a friend request but as a non-premium member you're only allowed to send out two friend requests per day.
3. The other user blocked you.
4. There's a technical glitch. Do you italki via the app or the website?
Please check if you received my message. If not, contact the italki team and let them check what the problem is.