Free quantity Goods SC Supplier not editable - ME21N

Hello All,

I am looking for the free quantity goods scenario in ME21N. I created the PO in ME21N for the Free quantity Goods scenario. 10,000 qty is chargeable and 2% of 200 qty is free qty. The 10,000 qty is enable for the SC supplier But the free qty details of SC supplier not editable.

So anyone know the solution kindly guide me,