A: Basically, there are 2 main reasons on why you can't transfer apps. First, one of the things listed here is not completed. Second reason can be that your client didn't setup payments data yet, and can't accept the app.
Q: How can my client accept the app?
A: First ,he will get a notification of a pending app. He needs to login to his iTunes, go to Agreements, Tax and Banking, and confirm. This looks like this