What is the NW version of your front end server? If it is < NW 740 have you installed the Fiori UI central add-ons? Also does your back end system have the componen GBAPP_POAPPROVAL installed?
Please read the implementation steps carefully before configuring any Fiori app and follow each and every step, this will save you a lot of unnecessary troublesh efforts.
Regards,
Saurabh