And this Anti-Christ, whose name was Korihor, (and the law could have no hold upon him) began to preach unto the people that there should be no Christ.
Why couldn’t the law do anything about him? Well, their law is God’s law, aka the commandments and scriptures. And the commandments say
And if it seem evil unto you to serve the Lord, choose you this day whom ye will serve; whether the gods which your fathers served that were on the other side of the flood, or the gods of the Amorites, in whose land ye dwell: but as for me and my house, we will serve the Lord.
Choose you this day whom you will serve. You are able to choose, and Korihor chose. Was it the right choice? No. But he used his agency to choose. And there was nothing the law could do about it.