You do not have to file 2016. But You have to start a 2016 return and pay for it to unlock the prior years. If you are in the Free Edition you have to upgrade to Plus or higher. After you upgrade to Plus or higher you need to actually PAY for it. Go up to My Account and click on Print Center and try to print your 2016 return and it should make you pay for 2016.
https://ttlc.intuit.com/questions/1901301-how-do-i-pay-my-turbotax-online-fees
