Once an order is confirmed, it will create a delivery and attempt to procure the required items. Adding another item after confirmation, it will immediately run this same process when saved. This means the product will need a valid procurement rule such as a buy or manufacture route if it is a stockable product.

You will either need to make it a non-stockable product (consumable or service), or add a valid route in the products inventory tab or in the products' category.

For the route to be valid not only does it need to be selected, but a vendor will need to be added to the purchase tab if it has a buy route, or a bill of materials will need to be added for a manufacture route.

This link may help you 

https://www.odoo.com/documentation/user/11.0/inventory/routes/concepts/procurement_rule.html