Trade Security Act of 2021 This bill revises provisions related to the adjustment of U.S. imports due to a national security threat. The bill allows Congress to block a presidential import adjustment through a joint resolution. This provision retroactively applies to adjustments made on or after July 1, 2018. However, it shall not apply to specified steel and aluminum import adjustments. The Department of Defense (currently, the Department of Commerce) must investigate the national security threat of a U.S. import, and report to the President the results of that investigation. Commerce must provide recommendations regarding such imports upon request by the President.