Since you didn't file or pay for a 2016 program under the same account that you used for the 2015 return ... then you have 2 options.
1) Pay for the 2016 program to unlock the prior year returns
You have to start a 2016 return using the Free edition from this website - https://turbotax.intuit.com/personal-taxes/online/
(If you already have the Deluxe version or better chosen then you can skip the upgrade and go straight to the payment instructions)
You must enter some basic personal information in the Personal Info section of the program. Click Upgrade, located near the top of your screen. In the pop-up window, scroll down to the bottom, select TurboTax Plus, and click the Upgrade Now button. If you want to include a state program with your order click on State Taxes and complete some of the state section.
To pay for the upgrade so you can access your prior year tax returns, assuming that you signed onto your 2016 account with the exact same user ID you used for the prior returns -
To pay the TurboTax online account fees by credit card, without completing the 2016 return at this time, click on My Account at the top of the online program screen. Then click on Print Center. Then click on Print, save or preview this year's return. On the next page, to pay by credit card, click Continue. On the next screen it will ask if you want Audit Defense, if you do not want this option just click on the Continue button. The next screen will ask for all your credit card information so you can pay for the account.
2) Contact TT support and have them unlock the return since you used another account for 2016:
Use the following TurboTax website to contact TurboTax customer service/support
There will be some personal information gathered so a case can be created for you. A phone number will then be provided, specific to your case, to speak directly with a TurboTax agent.
Note: The Call option will not be available when TurboTax Support is closed.