What does is a non-inventory item?

A non-inventory item is something that you want to add to a sales order but don't want to allocate serialized inventory to. Often, this will be used as a service like Freight and by creating it at a non-inventory master item, you can add it to the sales order.

If you have a phyiscal item that is labeled as a non-inventory item, you should go into your master items, find the item and change the Inventory Type to Inventory.