I had the same problem, and I was scratching my head for several days. Their documentation is very poor. Finally, I had to send the parameters in Query params instead of the body in the Post Request.

Below is the example URL we are supposed to use when making the POST request with nothing in the Body.

https://api.ccavenue.com/apis/servlet/DoWebTrans?access_code=AV**********&command=orderStatusTracker&request_type=JSON&response_type=JSON&version=1.2&enc_request=692eeaa4505843cfafcb454fd8d6a379b398f7cc989900200f0062118689c4cdc0399301cee1c9a2b70ebeed8c4c9d46bb69303b2720a61bb1200ac2784ec0ab