I filled out my 2016 tax return, but never filed it. Is it still possible to file it?

[xmasbaby0]

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.

[Critter]

All prior year returns must be printed, ink signed and mailed in ... if the return was ready to file then get the PDF and print the forms to mail ...  

To access your prior year online tax returns sign onto the TurboTax website with the userID you used to create the account - https://myturbotax.intuit.com/

Scroll down and on the section Your tax returns & documents click on Show.  Click on the Year and Click on Download/print return (PDF)