This means that the person you tried to send a message to doesn't have a version of Crewbuddy capable to receive messages yet.
This is because of one of two reasons:
1) They haven't updated to the latest version of Crewbuddy. They just need to go to the App Store and update CrewBuddy.
2) They have an Android. We're working hard to make CrewBuddy messaging available to Android users.