How can I analyze a failed IDoc when the status is "51" and the message is " application document not posted"?

In table EDIFCT there is the combination of message type and IDoc type assigned to a function module to processs the IDoc. Find this function and put a break-point in it. Then reprocess the IDoc with BD87 (or the ABAP RBDINPUT to save an extra step) and debug from where the procedure stops. From here, you are in the application and can to see what goes awry.