The only way to do a 2020 tax return is with the downloaded TurboTax software, which is what you bought. But you cannot install or run the downloaded TurboTax software on a phone or other mobile device. You have to install it on a computer. If you bought the Windows download you have to install it on a Windows computer. If you bought the Mac download you have to install it on a Mac computer.
The TurboTax mobile app uses TurboTax Online, but TurboTax Online is only for the current tax year, which is now 2023, so you cannot use the mobile app for 2020.
Minimum system requirements for TurboTax Windows software
Minimum system requirements for TurboTax Mac software
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