If you're following the tutorials, I'm fairly certain that your client.cpp file is trying to subscribe to a topic of type AddTwoInts from the tutorials. If this is the case, it means that your package cannot find the necessary message description. In order to access this information, you must either re-define the message type in your package as per the tutorials, or add the beginner_tutorials package as a dependency in your package. Do this by adding the following to your manifest.xml:
<depend package="beginner_tutorials" />