My friends, I am going to use an everyday example: when two people agree on a matter and sign an agreement, no one can break it or add anything to it.
Now, God made his promises to Abraham and to his descendant. The scripture does not use the plural "descendants," meaning many people, but the singular "descendant," meaning one person only, namely, Christ.
What I mean is that God made a covenant with Abraham and promised to keep it. The Law, which was given four hundred and thirty years later, cannot break that covenant and cancel God's promise. 1
References for Galatians 3:17
For if God's gift depends on the Law, then it no longer depends on his promise. However, it was because of his promise that God gave that gift to Abraham. 2
References for Galatians 3:18