If you were using the current online software, you used the wrong software. The online program is only for a 2017 return. To complete a 2016 return, you will need to purchase and download the 2016 desktop software, and then print, sign, and mail your 2016 return.
To file a return for a prior tax year
Only a 2017 return can be prepared online and e-filed.
If you need to prepare a return for 2014, 2015, or 2016 you can purchase and download desktop software to do it, then print, sign, and mail the return(s)
https://turbotax.intuit.com/personal-taxes/past-years-products/
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
