Stop Debt Collection Abuse Act of 2021 This bill applies certain consumer protections regarding debt collection to debt owed to a federal agency and to debt buyers. A federal agency that is a creditor may not sell or transfer a debt to a debt collector until 90 days after the obligation becomes delinquent or defaults. Specified notice to the consumer of such a sale or transfer is required. A collector of debt owed to a federal agency may not collect any interest, fee, charge, or expense that is (1) unreasonable in relation to actual costs, (2) not authorized by a contract between the debt collector and the federal agency, or (3) greater than 10% of the amount collected. The Government Accountability Office must report on the use of debt collectors by federal, state, and local government agencies.