Why can't the law save a person who is in the flesh?
Because when a person is in the flesh, under old Adam, the law's motions of sins work in the body's members to bring forth fruit unto death. The law can only produce death in that condition, for the wages of old Adam is eternal death, so the law cannot give life or salvation.
Les teaches that when we were in the flesh, or under old Adam, the motions of sins which were by the law worked in our members, in this body of flesh, to bring forth fruit unto death. That is all it can work for. The wages old Adam pays, who is under the control of Satan, is eternal death. But the flip side is that we are delivered from the law, being dead wherein we were held under old Adam and the curse, so that we should serve in newness of spirit, under the control of the Holy Spirit, the Word of God, and the Creator God Himself, and not in the oldness of the letter or the law.
For when we were in the flesh, the motions of sins, which were by the law, did work in our members to bring forth fruit unto death.