If you did not file a 2020 return, then use a Zero for your 2020 AGI.  If you did file a return in 2020, the following will help you find your 2020 AGI.  You can also just try the Zero for the 2020 AGI first.

How do I find last year's AGI?

https://ttlc.intuit.com/community/agi/help/how-do-i-find-last-year-s-agi/00/25947

How do I get my AGI if I don't have a prior year tax return?

https://ttlc.intuit.com/community/agi/help/how-do-i-get-my-agi-if-i-don-t-have-a-prior-year-tax-retu...