Follow the SPRO menu below.
First you need to go to "Valuation Class" and assign an Account Category Reference ("ARef") to the Valuation Class.
Then go back and click on "Material Type/account category reference" and assign the Material Type to the "ARef" that you assigned to your Valuation Class in the previous step.
Menu: SPRO->IMG->Materials Management>Valuation and Account Assignment>Account determination->Account determination without wizard-->Define valuation class
