Why can't the inheritance come by the Law instead of by promise?
The inheritance cannot come by the Law because the Law cannot fulfill God's promises; it only condemns. God's covenant with Abraham was based on promise—I will, I will, I will—and if the Law did the job, it would no longer be promise. The Law was added later for a reason.
Les teaches from Galatians 3:18 that if the inheritance is of the Law, then it is no more based on promise. But God dealt with Abram by promise: I will, I will, I will. Les explains that the Law could not fulfill any of those promises; all the Law could do was condemn Israel. It did not fulfill any promises per se, so if the Law was going to do the job, then it would no longer be a promise. God gave the Covenant based on promise. Then verse 19 asks, wherefore then serveth the Law? It was added. Les reminds us that God does not do anything without a valid reason and gave the Law for a reason. He also notes that from Adam to Moses was about 2,500 years without a written Law: 2,000 years to Abraham, then another 500 years (430 in round figures) until the Law was given at Sinai. The Law was added later and did not replace the promise-based inheritance.
For if the inheritance be of the law, it is no more of promise: but God gave it to Abraham by promise.