HTTP 400 · type invalid_request_error · retryable false

defaultMessage Session messages can only be sent within the 24-hour customer care window.

outside_24h_window

Session (free-form) send was rejected because the customer care window is closed.

What to do

1. Use an approved template message instead of a session message.
2. Wait until the customer messages you again to reopen the 24-hour window.
3. Confirm you are addressing the same phone / conversation as the last inbound.
4. Do not retry the same session payload until the window is open — it will keep failing accept with no Message row.