You filed your 1120-S late (calendar year S corporation returns were due 3/15/2021).
Generally, an S corporation must file Form 1120-S by the 15th day of the 3rd month after the end of its tax year. For calendar year corporations, the due date is March 15, 2021.
See https://www.irs.gov/instructions/i1120s#idm140355077348880
