Hi Shrinivas,
We had some issues with using Postman for load testing one of our Kentico websites. The main cause was missing User-Agent header. Could you make make sure your Postman requests look as close as possible to real live requests? I mean with correct accept and user-agent headers at least.
Also, your error screenshot shows that the error is happening in Kentico Web Analytics module. To troubleshoot it further you can disable in in Settings application.