Diagnosis
In the transaction "Enter Non-Allocatable Activities", you enteredbusiness process &V1&, which has allocation category &V2&. However,because the document already has allocation category 4(non-allocatable) you cannot use the given allocation category in thisdocument.

System Response
Within a document, you can use only the same allocation category,either category 3 (indirect allocation) or category 4 (non-allocatable).

Procedure
Enter only allocation category 4 for this document, or delete alldocument items and repeat your entries.