To access the prior year online tax returns the requirement is that you upgrade the current year online tax return to the Plus edition and then pay for the upgrade.
See this TurboTax support FAQ for when access to TurboTax online tax returns expire - https://ttlc.intuit.com/community/prior-year-return/help/when-does-access-to-my-prior-year-turbotax-...
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.
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