This error message indicates that the SMS failed to send because the notification does not contain any message content.
To resolve this issue, go to the Notifications section by opening the following link:
Change the Channel filter to SMS.
Review all enabled notifications and make sure that each one contains a message that can be sent to the passenger.
If a notification is enabled but the message field is empty, either add a message or disable that notification.
Once all enabled notifications contain a message, or empty ones are disabled, the Envelope app will be able to send SMS notifications to passengers successfully.