Well, you could buy 2018 desktop software, which also qualifies to unlock a 2016 return, but that is more expensive. The minimum to unlock prior returns is PLUS at 19.99. To do that you have to minimally start a 2018 online return in Free Edition and pay 19.99 for a PLUS upgrade.
Was your 2016 return prepared in Online TurboTax, and have you located it in your account in the section "Your Tax Returns & Documents"? And is access locked and it asks you to pay?
If so, the method to automatically unlock it is to minimally start a 2018 Free Edition return in the SAME account, just long enough to upgrade to PLUS and pay the 19.99 by credit/debit card. You don't actually have to prepare the 2018 return. Have you already started a 2018 return?
We can tell you all the steps on how to start one, upgrade, and pay.
Be aware there's a free option if a tax return transcript would suit your purpose. You may be able to immediately download a 2016 return transcript at the IRS website at the link below. If you don't qualify to open a free IRS account there, you can order one to come by postal mail, which normally takes 5-10 calendar days.
<a rel="nofollow" target="_blank" href="
http://www.irs.gov/Individuals/Get-Transcript">http://www.irs.gov/Individuals/Get-Transcript</a>