Unable to schedule a message in JetStream – BadRequestError: message schedules target is invalid

[Atanusaha143]

I am trying to schedule a message in NATS JetStream Message Scheduler  using Python (nats-py) but getting the following error:
nats.js.errors.BadRequestError: nats: BadRequestError: code=400 err_code=10190 description='message schedules target is invalid'

async def schedule_publish(self, stream: str, subject: str, message: dict, schedule_timestamp: str) -> None:
    nc = self._broker.nc
    if nc is None:
        raise RuntimeError("NATS connection not attached. Call broker.attach_nc(nc) first.")
    js = nc.jetstream()
    payload = json.dumps(message).encode()
    headers: dict[str, str] = {
        "Nats-Schedule": f"@at {schedule_timestamp}",
        "Nats-Schedule-Target": subject,
    }
    await js.publish(subject=subject, payload=payload, stream=stream, headers=headers)

Stream Info:

• Stream: test_stream
• Subjects: test_subject
• Replicas: 1
• Storage: File
• Retention: WorkQueue
• Allows Schedules: true

Issue Observed:

• The scheduled publish fails with the BadRequestError.
• The stream is configured to allow schedules (Allows Schedules: true).

Observations / Questions:

• The error message suggests the Nats-Schedule-Target may be invalid.
• I am using the same subject as the target (subject) in the header.
• Is there a specific format or requirements for Nats-Schedule-Target?
• Could the WorkQueue retention policy or ack mode affect scheduled messages?

Request:
Any guidance or example on correctly scheduling messages in JetStream via Python would be appreciated. Especially regarding:

• Correct Nats-Schedule-Target usage
• Requirements for the stream subject and retention
• Any gotchas with WorkQueue retention streams