Law and Perfection
Paul is not arguing that the Mosaic covenant considered as a covenant taught that perfect obedience would actually lead to justification. Rather, in response to the Galatian troublers, Paul insists that if one is going (foolishly) to turn to the law for justification one needs to read what the law actually says. Although the Mosaic covenant's sacrificial system provided forgiveness for transgressors, the legal demands of the law itself can do nothing but curse those who do not obey them perfectly.
Ben Dunson, "The Law Evidently Is Not Contrary to Faith": Galatians and the Republication of the Covenant of Works, Westminster Theological Journal, 79 (2017), 250