To access a prior year online tax return and you did not use TurboTax for the 2017 tax return or you did not pay for one of the 2017 online editions of either Plus, Deluxe, Premier or Self-Employed, you will need to start a 2018 Free online tax return and then Upgrade and pay for the Plus edition ($19.99). You do not have to file the 2018 tax return only pay for the upgrade.
Go to this TurboTax website for the 2018 online editions - https://turbotax.intuit.com/personal-taxes/online/
You have to sign onto the 2018 account using the exact same User ID you used to create the prior year online tax return. Enter your personal information in the 2018 Free edition. On the left side of the screen click on Upgrade. Scroll down and select PLUS More Help & Tools. Click on Upgrade Now (if Plus is grayed out then you are already using Plus and only need to pay for the upgrade)
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.
there is no link to the PLUS membership
You will not be able to upgrade to the Plus edition for tax year 2018 since the TurboTax online editions for 2018 have closed to make way for the 2019 online editions. Once the 2019 online editions become available in December, at that time you can start a 2019 return and upgrade to Plus.
You can also get free transcripts of your federal return from the IRS - http://www.irs.gov/Individuals/Get-Transcript
For a fee of $50 you can get a complete federal tax return from the IRS by completing Form 4506 - http://www.irs.gov/pub/irs-pdf/f4506.pdf
Or contact TurboTax support during business hours and see if there are any other options available for accessing a prior year online tax return.
Support is open from 5am to 5pm Pacific time, Monday thru Friday
Use this website to contact TurboTax support during business hours - https://support.turbotax.intuit.com/contact/
Support can also be reached by messaging them on these pages https://www.facebook.com/turbotax/ and https://twitter.com/TeamTurboTax