AWS Textract API UnknownOperationException from Postman REST API

Hi,

I am trying to use the Textract REST API using POSTMAN but not having any luck getting it working. I am receiving a Bad Request. Initially my Authentication was failing but it seems I have passed that now using Access and Secret key by setting an IAM user with full permission to Textract service.

I checked below steps in related post:

1. Obtain the access key and secret key for the IAM user. - DONE

2. Open Postman and create a new POST request. - DONE

3. In the Authorization tab, select "AWS Signature" from the TYPE dropdown menu. - DONE

4. In the Access Key field, enter the access key for the IAM user. - DONE

5. In the Secret Key field, enter the secret key for the IAM user. - DONE

6. In the AWS Region field, enter the region where you want to make the API call (e.g., "us-west-2"). - DONE (Used Sydney Region)

7. In the Service Name field, enter "textract". - DONE

8. In the Request URI field, enter the appropriate endpoint for the DetectDocumentText action. Add a JSON payload to the Body of the request with the necessary attributes and values to call the API. NOT WORKING

I am finding the issue here and am unable to get it working. I receive UnknownOperationException. I have attached the screenshots below for reference. Not sure what I am doing wrong. Any help I can get would be great. Thanks.