Access to prior-year returns depends on whether you've begun or filed your 2018 return using TurboTax. The TurboTax FAQ below will explain how you should proceed to unlock your 2016 return. Keep in mind that you'll need to make payment for, at a minimum, the PLUS feature in order to access the prior-year returns.
https://ttlc.intuit.com/community/upgrading/help/how-do-i-purchase-turbotax-plus-so-i-can-get-my-tax-file/00/26445
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