The error message (Error: API Error: {"message":"Shipment Document Validation Error(s):\nThe order does not allow a shipment to be created.\nYou can't create a shipment without products."}) when dispatching a Magento order on Mintsoft occurs because the shipment for the order has already been manually confirmed.
To resolve this, check Magento to verify if the shipment has already been processed before attempting to create a new one.