Solidworks did not boss-extrude lofted model

Dear all,

I am having difficulty in creating mold for the part of the box that can fit well with the curvy part of the lofted model. According to some YouTube tutorials I can choose Cavity feature, but that feature itself greys out. Therefore, how can I create the cavity like I want? I also attached the part file of this problem. Thank you very much for any of your advices!