SAP Note 23160 - No inventory management of material type KMAT

Symptom:

Inventory management is not possible for the material type KMAT in the standard delivered by SAP.

Solution
A) Inventory management in special stock
Material of the material type KMAT can only be managed in the special stocks E and Q with the value on the cost collector.

B) Creating a stockable type
Create a material master record (material type FERT) for the configured material (KMAT) that is to be posted to a warehouse.
On planning screen 2 enter a link in the field Standard product to the material which can be configured (material type KMAT). Stocks (storage types) can be managed for this actual variant of KMAT.

Further information: Chapter 7 of the manual Configuration of Variants