Custom Web Service Error ISG_SERVICE_EXECUTION_ERROR

I have developed a custom rest web service in EBS 12.1.3 and DB 12.1.0.2.0. After exposing my custom web service, I am invoking it from postman but I am getting following error:

{

  "ISGServiceFault": {

   "Code": "ISG_SERVICE_EXECUTION_ERROR",

   "Message": "Error while executing service",

   "Resolution": "System error, please see service log trace for details",

   "ServiceDetails": {

   "ServiceName": "InsertUserData",

   "OperationName": "insert_users_data",

   "InstanceId": "0"

  }

  }

}

Any suggestion or tip will be highly appreciated. Also how can I view service log?