Even if you have enough messages available to send in the current month, you may receive a 429 Too Many Requests error with the message You have reached your monthly limit. when you try to send a message while another message is being delivered.
This occurs because, when sending messages via the LINE Official Account Manager or the Messaging API, the number of messages scheduled for delivery is temporarily reserved from the start of the sending process until the actual number of messages is determined. Depending on your message delivery settings, the number of reserved messages may be based on the target reach, regardless of the actual number of recipients. This can temporarily exhaust your remaining message count.
If your remaining message count for the current month is temporarily depleted due to this reservation, sending another message may fail. Once the delivery in progress is complete and the number of messages sent is confirmed, you can try sending the message again if you still have messages available for the current month.
For more information, see Note regarding the number of remaining messages to be sent during the current month in the Messaging API reference.