There is an error message that is displaying on the messaging screen underneath the message that was attempting to send to the customer. The error message says "ERROR SENDING MESSAGE: Carrier Violation".
This message means that the cell phone carrier has blocked that message from coming through to the customer’s phone, similarly to how email providers block spam. This should not be a very common occurrence. If a customer is unable to receive the pre-written text from your store you can try to manually type a different message to that customer that relays the same information as the pre-written message.
<%= heading %>