No contract items of category & allowed

What causes this issue?

The contract item specifies the item category "material group". You
cannot enter any conditions for this item 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?

INCLUDE NA_EINGABE_PRUEFEN LANGUAGE EN

Error message extract from SAP system. Copyright SAP SE.