Commission on Domestic Terrorism Act of 2021 This bill establishes in the legislative branch the National Commission on Domestic Terrorism. The commission shall investigate relevant facts and circumstances relating to the federal government's failure to respond to and prevent acts of domestic terrorism in the past 20 years, including relating to the attack on the Capitol on January 6, 2021; identify, review, and evaluate the lessons learned from such acts regarding the structure, coordination, and management arrangements of the federal government, and, if appropriate, state and local governments and nongovernmental entities, relative to detecting, preventing, and responding to domestic terrorist attacks; and report to the President and Congress.