The current online software is ONLY for 2019 returns --which is what everyone is now preparing for the 2019 tax year that just ended. You cannot use online for any other year. And you have until April 15, 2020 to file your 2019 return. In fact, the IRS is not even accepting 2019 returns yet. They will start accepting them January 27. 2020.
Prior year returns can only be done using desktop software and the returns must be mailed in.
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.