Specify a subitem category

What causes this issue?

A subitem (set) has no subitem category. The subitem category is
determined by the respective FS item (by default) or is specified using
the F4 possible entries for the subitem (set).

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Repeat the entry using the F4 possible entries for the subitem.

Error message extract from SAP system. Copyright SAP SE.