Audit the Pentagon Act of 2021 This bill requires the Department of Defense (DOD) to reduce spending for any department, agency, or other DOD element that DOD determines has not achieved an unqualified opinion on its full financial statements for the calendar year ending during that fiscal year. Specifically, the amount available to such department, agency, or element for the fiscal year when the determination is made must be 1% less than the amount otherwise authorized to be appropriated. Amounts unavailable to such department, agency, or element must be applied on a pro rata basis against each program, project, and activity. DOD must deposit such unavailable amounts in the general fund of the Department of the Treasury for purposes of deficit reduction.