You cannot file a prior year return with the Online version, it is only for the current tax year (2019).  You would need the  Desktop software for prior year returns.  Prior year returns cannot be e-filed with TurboTax.  You can also go to the IRS website & your States website for 2018 forms to fill out.

For tax years 2018, 2017, or 2016

You'll need to purchase and download the TurboTax software for PC or Mac as TurboTax Online and the mobile app are only available for the current tax year.

After installing the software, open it and follow the onscreen instructions.

Prior year returns must be paper-filed, as e-filing is no longer available. We'll walk you through that process when you get to the File section in the software.