When you try to download the 2016 return, does it tell you that access is locked and that you have to pay?
Are your 2016 and 2018 returns in the SAME account? You can pay for the 2018 return in advance to unlock the prior return. We can tell you how that doesn't involve the FILE section.
The minimum to unlock a prior return is 2018 Free Edition upgraded to PLUS at 19.99. But you have a problem. You appear to be using "Live Basic" which is 79.99 for a Federal return and 34.99 for a state return. If Live Basic is the product you intend to use for 2018, we can tell you how to pay for it.
FAQ: What is TurboTax Live?
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Otherwise, if you are in that "Live Basic" product by accident and only wanted Free Edition with PLUS, we can tell you how to downgrade to Free Edition. And then how you can upgrade from Free Edition to PLUS, and pay for it a way that is easier to understand than TurboTax instructions. You won't even need to go the FILE section with my instructions.