The correct answer is: That slaves and freed blacks were not citizens of the United States.
In the Dred Scott v. Sandford case (1857), the U.S. Supreme Court ruled that African Americans, whether enslaved or free, could not be considered citizens of the United States and thus had no right to sue in federal court. The Court also ruled that the federal government had no authority to regulate slavery in the territories, effectively nullifying the Missouri Compromise.