Issue with Kendra query API

We are trying to execute kendra query API, but its return error like this,

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

Please help us to solve this.

POST: https://kendra.us-east-1.amazonaws.com

Request Body :

{ "IndexId": "xxxx-xxxx-xxxx-xxxx", "QueryText": "string", "QueryResultTypeFilter": "QUESTION_ANSWER" }