Why couldn't the Law fulfill God's promises to Abraham?
The Law could not fulfill God's promises to Abraham because its only function was to condemn Israel; it did not provide inheritance or fulfill promises itself. If inheritance came by Law, it would no longer be based on promise, but God gave the covenant to Abraham on the basis of promise.
Les explains that the Law was not part of the promises and certainly not part of grace. It was added later—2,500 years after Adam and roughly 430 years, or about 500 in round figures, after Abraham. Its purpose was not to fulfill the promises, because all the Law could do was condemn Israel. It didn't fulfill any promises per se. If the Law was going to do the job, then it would no longer be a promise. But God gave the Covenant to Abraham based on promise, promise, promise.