If you are using the online version of TurboTax, you can not enter a prior year tax return. Period. End of Story.
If you are using the CD/Desktop version of TurboTax that you physically installed on your MAC or Windows computer, then to enter your 2019 tax return you would have to purchase and install the TurboTax 2019 CD. when done, you would have to select the "print and mail" option in order to make the program think you actually filed the return.
Overall, there's really no need to enter a prior year tax return. If you are just now starting your 2020 taxes you will need a full copy of your 2019 tax return in order to deal with things like carry overs and depreciation if those items are applicable to you. The 2020 program will specifically ask you for things like carry overs, NOLs, depreciation, and much more. You would get that information from your 2019 tax return.
Generally speaking, in the first year you start using the TurboTax program, it's a bear because you have to enter *everything* one keystroke at a time. But after that, the program retains the history it needs provided you don't delete it.
Overall, first a first time user of TurboTax I do not recommend the online version of the program. It's not what I would call "user friendly" at all, for a first timer. Additionally, it lacks many of the features of the CD version, such as the ability to go into forms mode. Assuming you have already filed your 2020 tax return via other means, it would not be a bad idea to purchase the 2020 version of the CD to learn with. You enter data from your printout of the 2020 return you already filed by other means. Then you learn by doing, as I've found that for us humans, learning through osmosis just flat out never works. 🙂
