Of course, God's Eternal Law was not nailed to the Cross! But the Law of Moses was, in fact, nailed to the Cross because Jesus himself was dying *under the Law. *
He was dying, therefore, under very specific conditions, in order to free Israel from a debt they could not pay under the Law of Moses. All of their sacrifices and offerings fell short of providing relief from their Sin Debt and could never result in Eternal Life.
Jesus came to fulfill the Law of Moses--he did not come to destroy it. But he did come to end it by providing, in its place, his own eternal atonement.
While Israel remained under the Law, which is when Jesus said this, they were, of course, still required to follow the Law. But Jesus made it clear that the righteousness of the Pharisees would be insufficient, as a Law-based righteousness, to lead to eternal righteousness.
Something coming from himself would have to exceed it, which is what he provided through his resurrection and through his deliverance from the system of Law that never fully atoned for them. Thus, Jesus ended the Law by fulfilling what it had meant to portend, namely a greater righteousness that leads to final salvation.
Mat 5.17 "Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them. 18 For truly I tell you, until heaven and earth disappear, not the smallest letter, not the least stroke of a pen, will by any means disappear from the Law until everything is accomplished. 19 Therefore anyone who sets aside one of the least of these commands and teaches others accordingly will be called least in the kingdom of heaven, but whoever practices and teaches these commands will be called great in the kingdom of heaven. 20 For I tell you that unless your righteousness surpasses that of the Pharisees and the teachers of the law, you will certainly not enter the kingdom of heaven."
All of this was fulfilled at the Cross, which ended the system of Law, or the righteousness of the Pharisees. The entire realm of Creation was focused upon the necessity of Man's redemption at the Cross, and could never complete its epoch unless Christ died for God's creature, Man.