Diagnosis
You are entering a new BOM item and have entered a Resulting itemcategory for this item.
The resulting item category is used to categorize class items accordingto defined criteria, to enable you to enter item-specific data.
As you are not entering a class item, you cannot enter a resulting itemcategory.

Procedure
Delete your entry in the Resulting item category field.
If you want to enter an item with a class item category, correct yourentry in the Item category field.