The law was temporary!

“Why then the law? It was added because of transgressions (sin; violation)…

So then, the law was our guardian until Christ came, in order that we might be justified by faith. But now that faith has come, we are no longer under a guardian, for in Christ Jesus you are all sons of God, through faith.”
Galatians 3:19; 24-26 ESV (Brackets mine)
 
The word “added” shows us that law was not a part of God’s original plan. The law (the covenant which came through Moses) was “added” to the Covenant of Promise that God had already made with Abraham. The purpose of this was to expose sin and show that self-salvation could never be attained. The law prepared us for the covenant of grace (through Jesus) in which we need to let go of our effort for acceptance and blessing, and trust in our Saviour.
 
“So then, the law was our guardian until Christ came”
Galatians 3:24 ESV
 
The word “until” shows us that the law was a temporary situation “until Christ came”. Jesus came to fulfil the law; complete it and bring it to an end.
 
“For Christ is the end of the law for righteousness to every one that believeth.”
Romans 10:4 KJV
 
It is not possible to obtain right standing (righteousness) before God based on our effort and obedience to the law. It’s only by faith (believing) in what Christ has done for us.
 
“I do not frustrate the grace of God: for if righteousness come by the law, then Christ is dead in vain.”
Galatians 2:21 KJV
 
If you could be right with God by obeying and living by the law (the Old Covenant), then it would frustrate (void, stop the flow or reject) the grace of God (the free gift of undeserved favour) and it would mean that Christ’s death had no purpose.