Why doesn't my 2019 return automatically fill in my return for 2020?

You will need to make sure you are in the same account that was used last year. It is possible to have a different username with the same email and start a new account.

For more information, see: How many Intuit Accounts do I have with TurboTax?

How do I transfer last year's return into TurboTax Online?

You have to use the same exact User ID and password as you did for 2019.