You can't prepare 2019 and 2020 returns online with TurboTax. Only a 2021 return can currently be prepared in Online TurboTax. Tax years 2019 and 2020 would have to be prepared in desktop software for each particular year.
You can purchase 2019 and 2020 desktop software programs directly from TurboTax as downloads at this page and install each one on your computer:
https://turbotax.intuit.com/personal-taxes/past-years-products/
Or you may be able to find the 2019 and 2020 software cheaper at a reputable authorized retailer like Amazon, but even there be sure it is a legitimate authorized product from Intuit sold directly by Amazon itself, since there's no way to be sure all third-party sellers on Amazon are legitimate.
NOTE: Be aware there are "too-good-to-be true" Internet scam sites selling fake or pirated software.
Minimum system requirements for Windows version:
https://ttlc.intuit.com/questions/1901571-minimum-system-requirements-for-turbotax-windows-software
Minimum system requirements for Mac version:
https://ttlc.intuit.com/questions/1899173-minimum-system-requirements-for-turbotax-mac-software
The 2019 and 2020 returns cannot be efiled. Each one will need to be printed, signed, dated, and mailed. It's a good idea to mail by a method that has tracking, such as certified mail, priority mail, etc. And you should mail the returns for the different tax years separately in different envelopes. Do not combine returns for different years in the same envelope.
If you have both Federal and state returns, you need to mail each Federal return to the IRS, and each state return (if applicable) to the state tax agency.
