Green Bus Tax Credit Act of 2021 This bill allows a manufacturer a zero-emission bus tax credit through 2026. The credit is equal to 10% of the sales price of a zero-emission bus. The credit only applies to the first $1 million of a vehicle's sales price and to buses used primarily in the United States and its possessions. The bill defines zero-emission bus as a motor vehicle that has a gross vehicle weight rating of not less than 14,000 pounds, is not powered or charged by an internal combustion engine, is propelled solely by an electric motor that draws electricity from a battery or fuel cell, and is designed to carry 15 or more passengers.