You can only do the last 3 years with Turbo Tax. Online is only good for the current year. For prior years you need to buy the Desktop program. You have to buy each year separately. You can buy the last 3 years of Turbo Tax here,
https://turbotax.intuit.com/personal-taxes/past-years-products/
You will need a full Windows or Mac to install it on.
How to prepare a prior year return
https://ttlc.intuit.com/community/using-turbotax/help/how-do-i-prepare-a-prior-year-tax-return/00/25...
Or if you want to file for free and have a simple return you can get the blank forms from the IRS and fill them in by hand. I have a list of some of the basic ones I can post direct links to.
Oh you would not be able to use the 2020 return you already started. It might let you download the .tax2020 data file but you couldn’t use it since they don’t sell the 2020 program to open it.
