Field MBEW-BWTTY in Material Master not available for changes

I have created a new plant. When I try to expand a material, which has split valuation, for this new plant, the field MBEW-BWTTY (valuation category) in the finance 1 view at the material master doesn´t allow any change and in this case I´m not able to establish the valuation category for the material in this new plant, to fix the valuation type of the split valuation. Thanks for any help.