Final answer:
The main argument against granting Dred Scott his freedom was that the Supreme Court ruled that as an enslaved person, he was not a citizen and therefore did not have the right to sue for his freedom. The Court's majority opinion stated that Scott's time spent in free states and territories did not make him a free man.
Step-by-step explanation:
The main argument against granting Dred Scott his freedom was that the Supreme Court believed that as an enslaved person, he was not a citizen and therefore did not have the right to sue for his freedom in federal court. Chief Justice Roger Taney, in the Court's majority opinion, stated that Scott's time spent in free states and territories did not make him a free man since Black people could never be citizens of the United States.