axpayers under regular category are filing GSTR 1 and GSTR 3B to report their transactions. GSTR 1 is for reporting of details of outward supplies including amendments if any and GSTR 3B is for reporting summary transactions for payment of taxes and claiming ITC.
Transactions relating to the tax period April 2020 to March 2021 may be rectified while filing returns for September 2021 (Due date for filing GSTR 3B for September 2021 is 20th / 22nd / 24th October 2021). No additional claim of missing ITC may be made available post filing of September 21 returns. You may lose the right to claim any eligible ITC which appears in your books of accounts and not claimed till filing of September 2021 GSTR 3B return.
Source Taxguru
