Until the 2019 Online version goes live sometime in December you will not be able to unlock a prior year return ....
Access to prior year returns.
You can get a free transcript from the IRS:
How do I get a copy of a return I filed using online TurboTax?
How do I get a copy of a return I prepared using desktop TurboTax?
How to upgrade to PLUS
In TurboTax Free Edition, you can purchase PLUS to unlock your prior-year .tax file.
The exact instructions will vary depending on the status of your 2019 TurboTax Online return (select one):
Note: While you still can, print a copy of your current year tax return and save it as a pdf, too. That way you will have it when you need it for school, a loan, etc. without going through a lot of aggravation to get it.
User justin_michael91 in the thread linked below said he was able to pay for PLUS this way:
Right after signing in, clicking on HELP in the top right, then scrolling way down to CONTACT US. Then following the prompts, entering a question, then choosing to phone Support. It required him to pay for PLUS to get phone privilege, and that unlocked the past returns.
Whether that will work for everyone or not, I don't know. Here is the thread where he described what he did: