Afghanistan War Commission Act of 2021 This bill establishes the Afghanistan War Commission. The commission must conduct a study of various matters related to the war in Afghanistan, including (1) U.S. actions immediately prior to the attacks on September 11, 2001; (2) U.S. actions during the invasion of Afghanistan; (3) peace negotiations involving the United States, the government of Afghanistan, and the Taliban; and (4) the U.S. military withdrawal from Afghanistan. The commission must annually report to Congress on the progress of its study. Within four years of the commission's first meeting, it must submit to Congress and the President a final report with its findings, conclusions, and recommendations to address any mistakes in the conduct of the war.