AWS API Gateway giving UnknownOperationException on integrating with other AWS services

Question

I am trying to put AWS API Gateway in front of the AWS Config service. Primarily to allow access on AWS Config service data to the wider audience without having them onboard on to the AWS ecosystem. The specific action which I am trying to access is DescribeConfigRuleEvaluationStatus. My issue isn't restricted to this action though.

I have set up API Gateway integration points as below:

But while testing it out, I am getting below error in the response body. Any thoughts on what could be the possible reason?

{
  "Output": {
    "__type": "com.amazon.coral.service#UnknownOperationException",
    "message": null
  },
  "Version": "1.0"
}