If you are using "Standard Costing" as your costing method in Inventory (Defined in the "Account Set"), then you will need to enter a Cost in the Standard Cost field. Otherwise, you caqn leave this field blank.

If you are not using Standard Costing, then you can enter an amount in "Recent Cost". This will be used as the cost of the inventory item if you sell it without receiving any quantity into inventory (driving the quantity on hand negative).

The actual (correct) cost for the item will be set when you receive some items into inventory.