As per section 16(4) of the CGST Act, you can claim your Input Tax Credit for a financial year till the due date of GSTR 3B of September of next financial year or date of filing of annual return, which ever is earlier.
So you can claim ITC of March 21 till the due date of GSTR 3B of Sep 2021 i.e. 20th October 2021 or date of filing of annual return for financial year 2020-21, whichever is earlier. You can claim this ITC in GSTR 3B.