Enabling Gateway orders with Gateway linked suppliers
Gateway Orders enables the PO to be directly posted into the suppliers' Gateway Account allowing suppliers to easily organise and manage the PO's that are sent.
To check if Gateway Orders are ON, go to the supplier profile in Suppliers > Connections. Find the Supplier and select Actions > Edit or Actions > View. You'll then be able to see in the 'Gateway Features' section whether 'Orders' is ticked On or not. 
If the supplier is linked through the Gateway and the Gateway orders functionality is ON you need to ensure any products you wish to order exist within the supplier Gateway. Otherwise the system will give you a warning message and highlight the products that do not exist - you will not be able to order the products until they exist in the supplier Gateway catalogue.
You will also not be able to accept or decline orders placed on the suppliers behalf. As the two systems are fully linked the supplier will need to complete their side of the process to enable the system updates.
See Gateway settings for more information on the Gateway features