Hi Benita,
GBAPP_APV_PO is the package for Approve PO. Your app is Approve Leave Requests but you get error before calling Approve Request. So you should set break point at TASKPROCESSING service.
Go to /IWFND/MAINT_SERVICE transaction and select the TASKPROCESSING. You see Service Implementation button at the lower right.
Regards,
Masa
SAP Rapid Innovation Group - RIG