My return gets rejected saying that my AGI from 2020 doesn't match-but I didn't file in 2020-I was under my parents and a high school student.

Since you did not file a 2020 tax return in the File section of the program you should have indicated that when asked.

Otherwise, enter a 0 (zero) for the 2020 AGI and e-file the 2021 tax return again.

I have tried entering 0 and it gets rejected. Is there a way for me to open my 2021 and fix the information that I entered wrong?

To revisit the 2020 AGI section in TurboTax Online:

1. Sign in and open your return, if you haven't already done so.
2. From the left menu, select File (tap in the upper-left corner to expand the menu on mobile devices).
3. Select Start next to Step 3 (if needed, revisit Steps 1 and/or 2 to activate Step 3).
4. Select I want to e-file.
5. Continue until you see the place to enter your 2020 AGI.

If you used a zero and it was rejected you will have to mail in the return ... in the FILE tab choose the mail in option. To get back into the account, if clicking on the fix now button doesn't work then scroll down and click on ADD A STATE to open the program back up.

First time filers have to mail in their first Form 1040.

Unfortunately, IRS is in chaos now and expect it to take up to a year to process a paper tax return.

@treyway44