PDF Services API – “INVALID_REQUEST_FORMAT” when calling Document Generation endpoint from Postman

I am trying to use the Adobe PDF Services API for Document Generation.

I have already set up my credentials in the Adobe Developer Console (OAuth Server-to-Server integration, PDF Services API is enabled).

What I am doing:

• Endpoint:

POST https://pdf-services.adobe.io/operation/documentgeneration

• Headers:

Authorization: Bearer <access_token>

x-api-key: <client_id_from_console>

Content-Type: application/json

• Body (raw JSON):

{

  "outputFormat": "docx",

  "templateContent": "<base64 encoded docx file here>",

  "jsonDataForMerge": {

    "Table1": [

      { "Col1": "AAA", "Col2": "Married", "Col3": "50000" },

      { "Col1": "BBB",  "Col2": "Married", "Col3": "100000" }

    ]

  }

}

Problem:

Every request fails with status 400 error:

{"error":{"code":"UNSUPPORTED_OPERATION","message":"Unsupported Operation."}}

Notes:

• The access token is valid (recently generated from Developer Console).
• x-api-key is set to my Client ID.
• Base64 encoding of the DOCX is correct (tested separately).
• Tried both Postman and Workfront Fusion, but same error persists.

Question:

Am I missing something in the JSON structure or headers?

Please can someone let me know ASAP