Hi Ankush,
Adding to Josh point, i will suggest, go bit by bit process,
Like first try to parse your request only by proving payload mapping file with single payload details like below
Payloads
Payload name=operation-req type=request key=abcdxyz
Section
Copybookfile
Your key is important and it should be available in the request.
This error sometime comes due to if there is no match in key properly.
If still you get error, might b try to check if your copybook file parsing into Xml is happening properly.