Diagnosis
The sales document category of the delivery type lets the system knowwhether it is dealing with an inbound or outbound delivery. Thisinformation is also on the item category level so that item categoriesare not used for an inbound and an outbound delivery simultaneously.You attempted to assign the item category to a delivery type that had adifferent sales document category.

Procedure
Choose an item category with the same sales document category as thedelivery type or change the sales document category in the deliveryitem category activity in Customizing.