Hi Diptendu
Content not allowed in prolog error is indicative that the content is not in XML format as expected.
You have selected the response format as XML. Maybe the response is also in JSON format. Did you try this using an external tool like Postman or Chrome's Advanced REST client?
You can also add logging (using MessageLoggerBean) at the receiver channel to log the content of the payload for the response to see what you are actually receiving. With this, it will help you design your interface accordingly.
Message Staging and Logging Options in Advanced Adapter Engine of PI/PO 7.3x/7.4
Regards
Eng Swee