Hi,
by saying 'once i save there where no effect, that check BOX still blank', do you mean you are unable to save the setting, or it is able to save but is with no effect at all?
BTW, since this setting is against a partner function within a partner procedure which should be assigned to an object (in your case, it is sales item category), could you plz check if the sales item category (you are doing practice with) is with the partner procedure where the partner function (sales employee) is with 'mandatory' checkbox on?