Can't change Default Category Set of Process Cost.

Hi,

     I couldn't change the default category set of Process Cost.  The message appeared that "APP-INV-05115: You must assign all items of this type to the new set first". My oracle application is version 12.2.3.    I perform the following steps as

     1. Create Key Flexfield Segment  Inventory : Item Categories

     2. Create Value at Categories Code

     3. Define Category Sets

     4. Assign this categories to all items

     5. Change  Default Category Set of Process Cost  with this Category but unable to set.  It appeared the message ""APP-INV-05115: You must assign all items of this type to the new set first".

     What is caused this problem?

Thank you