Hi,
Normally, a payment plugin will implement the event onAfterOrderConfirm from HikaShop's payment API:
www.hikashop.com/support/documentation/6...#onAfterOrderConfirm
In this event, it will get the order information and process it in order to generate some HTML code (for example, the HTML necessary to redirect the user to the payment gateway at the end of the checkout).
This error message will appear if your plugin does not output any HTML when this function of the payment plugin is called or if it doesn't have this function.