Diagnosis
You want to create a class item for a BOM. However, the status of thisclass does not allow the class to be used for selecting objects.
You can only enter classes with a status which allows selection. Thisis defined in Customizing (Classes -> Status) using the indicatorSelection allowed.

Procedure
If you want to enter this class as an item, first you have to changethe status of the selected class. Otherwise, select a class with astatus which allows selection.