Diagnosis
You want to create sub-items for the chosen items. However, this is notpossible.

System Response
The system checks the settings in the customizing step Define itemcategories. Whether sub-items are supported depends on the itemcategory.

Procedure
Since sub-items are not supported for the item category you selected,you cannot maintain sub-items.
If you need to maintain sub-items for this item category, contact yoursystem administrator.