To complete and file a 2018 and 2017 tax return using TurboTax you would need to purchase, download and install on a personal computer one of the 2018 and 2017 desktop editions from this website - https://turbotax.intuit.com/personal-taxes/past-years-products/
A prior year tax return can only be printed and mailed, it cannot be e-filed.
Each tax year has to be filed separately using the forms for the specific tax year. They cannot be combined in any way--do not even put them in the same envelope when you mail them. Software for past years is available back to 2016. Any earlier years can only be prepared on paper forms by hand--and those refunds have been forfeited now.
The current online program can only be used for a 2019 return--no other year. And if you did not file a 2018 return you will have trouble e-filing 2019. When it asks for your 2018 AGI try using zero. If zero does not work then you have to print, sign and mail 2019.
Online preparation and e-filing for 2018 is permanently closed.
To file a return for a prior tax year
If you need to prepare a return for 2016, 2017, or 2018 you can purchase and download desktop software to do it, then print, sign, and mail the return(s)
Remember to prepare your state return as well—if you live in a state that has a state income tax.
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/state received the return.
Note: The desktop software you need to prepare the prior year return must be installed/downloaded to a full PC or Mac. It cannot be used on a mobile device.
If you are getting a refund, there is not a penalty for filing past the deadline. If you owe taxes, the interest/penalties will be calculated by the IRS based on how much you owe and when they receive your return and payment. The IRS will bill you for this; it will not be calculated by TurboTax.