To file a 2019 tax return using TurboTax you must be using one of the 2019 desktop editions installed on your personal computer.
To change how the tax return is filed you need to click on File at the top of the desktop program screen and then select File by Mail.
The Online version is now only for 2020. Did you enter your 2019 info into the 2020 online version this year or did you start 2019 last year? If you entered it this year you have a problem and need to clear it out.
Did you start an Online return? How to finish a prior year online return
Or How to file a prior year