The Old Covenant was never revoked, even though it was made obsolete and replaced by a new and better covenant. The law must still be fulfilled but the right way now, by grace, by love, by communion with God, ‘apart from Whom we can do nothing’, John 15:5. We follow Jesus into this union/communion, and become transformed into law abiders, obedient because we want to, obedient because we love.
“Let no debt remain outstanding, except the continuing debt to love one another, for whoever loves others has fulfilled the law. The commandments, “You shall not commit adultery,” “You shall not murder,” “You shall not steal,” “You shall not covet,”a and whatever other command there may be, are summed up in this one command: “Love your neighbor as yourself.” Love does no harm to a neighbor. Therefore love is the fulfillment of the law.” Rom 13:8-11
From the Catechism:
121 The Old Testament is an indispensable part of Sacred Scripture. Its books are divinely inspired and retain a permanent value, for the Old Covenant has never been revoked.
The Law is powerless to justify us; it “kills” if we try to obey by our own efforts, apart from God, because we can only fail in the end, or put on a pretense of holiness at best.