Unfortunately, TurboTax Online doesn't have the ability to generate .ttXX files. You can only download a PDF of your completed return.
Please find below TurboTax FAQs to clarify the transfer of previous returns.
This TurboTax FAQ tells you how to save/print a copy of your tax return in TurboTax
Thank you for using TurboTax.
Why on earth does Intuit not provide complete interfacing of Turbotax files between platforms, including on-line, for year-to-year data transfer? I'm sure that last year when my wife came home with the on-line Turbotax 2019 version from Costco that I was able to transfer files from my computer based Turbotax2018 files into my online Turbotax 2019 tax files. This is ludicrous that Intuit does not provide compatibility in the other direction also. Last year when I saw that the copy my wife brought home was for on-line, I thought that that was the way everything was going and that there were no more CD / downloadable versions. Consequently, I went ahead and used the online version. Then this year I pick up my copy at Costco again, and this time there is a CD in it! Maybe Costco has both version but the packaging sure is not obvious!
Now I am in the same boat as hniles2019 I think. I am not able to transfer my on-line Turbotax 2019 files into my computer based Turbotax files for 2020!
Can I swap my CD / downloadable version for an online version? Please let me know as soon as possible.