You need your 2020 AGI on the 1040 line 11 to efile 2021.
How to find last year's AGI
https://ttlc.intuit.com/community/agi/help/how-do-i-find-last-year-s-agi/00/25947
Or You can request a transcript from the IRS
https://www.irs.gov/individuals/get-transcript
