National Flood Insurance Program Reauthorization and Reform Act of 2021 This bill generally revises the National Flood Insurance Program (NFIP) and reauthorizes the program through FY2026. The bill addresses NFIP coverage, cost, and availability, including by generally prohibiting the Federal Emergency Management Agency (FEMA) from raising certain premiums, surcharges, and fees more than 9% a year for five years; revising flood insurance coverage limits; establishing a means-tested program to provide financial assistance to low income households through policy discounts; and revising standards and certification requirements for flood insurance rate maps. The bill also revises administrative provisions of the NFIP, including by allowing for the continuous operation of the NFIP during a lapse in appropriations, and prohibiting the Department of the Treasury from charging FEMA interest for NFIP debt for five years. The bill sets forth requirements for Write Your Own companies related to reimbursements, agent commissions, and penalties for underpayment of claims. (A Write Your Own company writes and services federal standard flood insurance policies in its own name.) The bill establishes state or tribal government revolving funds for flood mitigation activities and also provides for loans, grants, and other incentives regarding mitigation.