You have to print and mail prior year returns. Did you use the Online version? Online is only good for 2019. So if you did them online you have a big problem. You have to buy and install the Desktop program on a full Windows or Mac computer to do prior year returns.
You can only buy the download for prior years. You have to buy a separate program for each year. They only sell the last 3 years. You need a full Windows or Mac computer to install it on. You can buy any version. All the Desktop programs have the same forms. You just get more help and guidance in the higher versions. Buy the download here https://turbotax.intuit.com/personal-taxes/past-years-products
If you want to file for free you can fill out the forms by hand.
Prior year returns can only be printed and mailed. When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s. Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS received the return. And don't forget state.
Important! Mail Federal and State in separate envelopes and mail each year in separate envelopes.
To complete and file a 2017 and 2018 tax return using TurboTax in 2020 could only have been done by purchasing, downloading and installing on a personal computer one of the desktop editions for each tax year from this website - https://turbotax.intuit.com/personal-taxes/past-years-products/
The 2017 and 2018 tax returns could only be printed and mailed, then could not have been e-filed.
The TurboTax online editions are only for tax year 2019. So if you used the online editions the IRS will be contacting you by mail questioning why you filed prior year tax data on a 2019 tax return.