2037: Reply target not ready.
The reply target has no acknowledged channel identity yet.
HTTP status: 422.
reply_to_message_id names a message this API accepted but the channel has not yet acknowledged, so a reply cannot be threaded to it. Nothing was sent.
Troubleshooting
Section titled “Troubleshooting”- Read the target message until its status is sent (its channel_message_id appears at the same moment), then resend the reply. message.sent is journaled when the send is accepted, not when the channel acknowledges it, so it is not the signal to wait for.