You also need to PAY for the 2018 program to unlock the prior years ... if you are already in PLUS or better then do not upgrade further just pay ...
To pay the TurboTax online account fees by credit card so you
can access your prior year online tax returns, click on Tax Tools on
the left side 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 Pay
Now. On the next screen just click on the Continue button. The
next screen will show a review of your Fees, click on
Continue. Complete the next screen to pay by credit card and click
Continue.