The Transform Configuration dialog box will only list messages that match the message type of your BTS mapping.
Typically, you have your mapping which maps between message type A and message type B. Here, you are selecting the destination message type. If your message is not listed here, it means it does not have the same message type as your BTS mapping expects it to be in it's destination schema.
Make sure you select the correct message type for your message in your orchestration or - if you are sure that this matches - double check the destination of your BTS mapping.
Either way, there will be a discrepancy between the two.