How can I sign in to my 2019 return, not 2020?

The current ONLINE program is for the 2020 tax year ONLY.   So what do you need to do with a 2019 return?  Look at it or prepare it ?  

Can't you find 2019?  It's common to end up with multiple accounts. First LOG OUT of whatever TurboTax account you're logged into right now. Then use this TurboTax account recovery website to get a list of user ID's for an email address. Run the tool against any email addresses you may have used
https://myturbotax.intuit.com/account-recovery/

If you used the Desktop CD/Download program then the only copy is on your computer and not saved or stored online.

You can also request a transcript from the IRS
https://www.irs.gov/individuals/get-transcript