You have to populate the task ID in your SAP ECC backend in table GBAPP_C_PO_TASK. While this doesn't seem to be documented anywhere in SAP Help, it is documented in the code as shown in the screenshot below.
Correction - I've checked the implementation guide for the app and setting up the task type is actually documented there. In your SAP ECC system you go to SPRO and follow this path: Materials Management-->Purchasing-->Purchase Order-->Approve Purchase Order App-->Specify Workflow Task IDs for Approve Purchase Orders App. This will effectively populate the table I mentioned in my earlier reply. Message was edited by: Lukasz Grochal