To access prior year tax returns you have to upgrade and pay for one of the TurboTax online editions of either Plus, Deluxe, Premier or Self-Employed.
If you used the TurboTax online Free edition for 2018 and upgraded past the Plus edition to Deluxe then you will need to contact TurboTax support for assistance in only paying for the Plus edition. After payment you will have access to all prior online tax returns through October 2020, provided you created the 2018 account using the exact same User ID you used to create the prior year accounts.
Use the key words billing issues when entering your problem on the TurboTax contact website.
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