Hello Sravanthi,
This is the expected error when we are performing mapping during implementing Query operation in SEGW.
All you need to do is you need to define a property as Key which is one among a field in the out response.
i.e., which ever tables of the FM your are mapping as output response, pick any one property from them and define it as a key.
This will resolve the error.
Regards,
Ashwin