The 2020 software doesn't do the transferring. The 2021 online software transfers the information from the previous year when you start your 2021 tax return. You need the .tax2020 data file that the 2020 software creates on your computer. See the following FAQ for instructions.
How do I transfer last year's return into TurboTax Online?
TurboTax Online for 2021 will only be available until October 17. If you cannot finish and file your 2021 tax return by then, you will have to use the 2021 download software (which can also transfer your information from 2020). Don't wait until the last minute.