Specify a subitem category

What causes this issue?

You have assigned a subitem set to the breakdown category. However, you
have not assigned a subitem category.

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?

Please assign a subitem category to the breakdown category or remove
the subitem set.

Error message extract from SAP system. Copyright SAP SE.