You would need to amend your 2016 tax return if you filed a 2016 tax return but forgot to include that W-2. If you did not file a 2016 return at all, then you could still prepare a 2016 return and print, sign and mail it.
To amend your 2016 return: https://ttlc.intuit.com/questions/2952222-how-do-i-amend-the-2016-taxes-i-filed-with-turbotax-online
Online preparation and e-filing for 2016 is permanently closed.
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/
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 received the return.
**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.**
