The error General Winfield Scott is referring to is the refusal by the Cherokee Nation to obey the agreement of New Echota.
The Treaty of New Echota was signed in 1835. In it, the Cherokee nation ceded its territory in the southeast and agreed to move west to Indian Territory. This treaty became the legal basis for the forcible removal act known as the Trial of Tears.
The Cherokke Nation rejected the treaty which led to the forceful removal in 1838. President Martin Van Buren directed General Winfield Scott to forcibly remove all the Cherokees who had not complied with the treaty and moved west. The error they committed is their rejection to move voluntarily during the allocated two-year-time.