This answer assumes your 2020 return was prepared in Online TurboTax. I've posted below how to normally get it, and also how to look for multiple accounts, if necessary. Any return prepared in desktop software will not be in an online account.
First see this:
FAQ: How do I access my prior year return?
https://ttlc.intuit.com/community/prior-year-return/help/how-do-i-access-my-prior-year-return/01/270...
Log in and go to the Tax Home. Scroll down the Tax Home page to the section "Your Tax Returns & Documents." Expand that section and choose the tax year 2020. Then you can choose to "Download/print the PDF", or if you just need the 2020 AGI, then once you choose 2020 there should be a link to "View adjusted gross income (AGI.)"
If you do all that, but there's no evidence of the 2020 online return in that section "Your Tax Returns & Documents", then you likely have multiple accounts and signed into the wrong one. Many people end up with multiple accounts.
You can have up to 5 accounts that use the same email address for notification purposes. A User ID may be an email address, but it doesn't have to be. It might be only part of an email address, or it can be anything at all.
To get a list of your User IDs, reset password, and recover account access, etc., you can use the tool at the link below. When using the Account Recovery tool, try using your phone number first if you can still access it. After that, if necessary, then run the tool on your email address(es) you can access.
NOTE: Before running the account recovery tool below, log out of all Intuit accounts including this user forum, or you might end up in a loop. Then clear your browser Internet cache, close your browser, then reopen it, and go to the link below. You may wish to copy this link so you can paste it into the new browser session.
https://myturbotax.intuit.com/account-recovery
If still no luck after running that on your phone number you can still access and email address(es) that you can access, here's another method:
Go back to the tool again, but this time leave the data field blank, scroll down a bit, then choose the small blue link that says "Try something else", and it will look you up by SSN and other parameters.
