You don't use the AGI from your 2020 amended return, you need the AGI from your original 2020 return.
To revisit the 2020 AGI section in TurboTax Online:
- Sign in and open your return, if you haven't already done so.
- From the left menu, select File (tap in the upper-left corner to expand the menu on mobile devices).
- Select Start next to Step 3 (if needed, revisit Steps 1 and/or 2 to activate Step 3).
- Select I want to e-file.
- Continue until you see the place to enter your 2020 AGI.
https://ttlc.intuit.com/community/agi/help/where-do-i-correct-my-agi-in-turbotax-online/00/26311
How do I correct my 2020 AGI in the TurboTax CD/Download software?
To correct your 2020 AGI in TurboTax, open your return and then:
- Select the File tab and continue until you reach the screen that says To continue, select options below
- Enter your corrected AGI in Enter last year's AGI here.
- Select Continue and follow the screens until you've re-transmitted your return.
