You cannot prepare a 2018 return with the Online version. The Online version is only to prepare the current tax return, which is 2019.
To file a 2018 return with TurboTax you need to download the 2018 Desktop software. Prior year returns cannot be e-filed with TurboTax, they must be mailed.
You can purchase the 2018 TurboTax CD/Download software which is available at at the following website
past-years' taxes page
