Yes you can still file your 2016 return but it cannot be e-filed. Only a 2018 return can be e-filed. E-filing for previous years is permanently closed. You have to print it, sign it and mail it. If you are getting a refund there is no penalty for filing after the deadline. If you owe tax due, you should expect a bill from the IRS for the penalty/interest.
To finish a prior year return started in TT online:https://ttlc.intuit.com/questions/2559403-how-do-i-finish-a-prior-year-return-that-i-started-in-turb
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 and/or your state 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.**
