Graph Beta API Start transcription not working

Issue
I am trying to start the transcription of a Teams call using the graph api http request. Refer the link below:
https://learn.microsoft.com/en-us/graph/api/call-starttranscription?view=graph-rest-beta&tabs=http

This api is returning a 400 bad request. Refer the request & response bodies below

Request

{
  "language": "en-us"
}

Response

{
    "error": {
        "code": "",
        "message": "The input was not valid.",
        "details": [
            {
                "code": "",
                "message": "The input was not valid."
            }
        ]
    }
}

Code Snippet
This is just a request from Postman

Expected behavior
Expecting a status 202 response body

{
  "@odata.type": "#microsoft.graph.startTranscriptionOperation",
  "clientContext": "clientContext-value",
  "status": "running"
}

Graph SDK (please complete the following information):

• Beta version

Call ID
Call Id: 08002980-9c3b-40cf-97e1-d9f4bd75ca0d
Chain Id: eea4773b-28cb-465a-9d67-b34568e59f49
Request-id': 'bb544e93-6eaf-4f8a-bc1b-6308bd2ee288',
Client-request-id': 'bb544e93-6eaf-4f8a-bc1b-6308bd2ee288'
2025-12-05T05:23:23 UTC

Logs
If required, please add logs from the SDK. (Please remove any PII from the logs before uploading)

Additional context
My scenario

1. PSTN user calling in the resource account
2. Microsoft Graph bot answers the call
3. M365 Teams user is invited via Graph API
4. M365 Teams user joins
5. Start transcript
   This issue is not happening to a specific call. It happens to all the calls

Start with the linked Microsoft Graph beta startTranscription endpoint and reproduce the request in Postman using the supplied language value and call scenario. Review the returned 400 response alongside the request IDs and timestamp, then check the linked logging guidance if SDK logs are needed. Done means identifying whether the request or endpoint behavior explains the consistent failure and recording the verified requirements or result.