You cannot file a previous years return with the Online version, the Online version is only to file the current years tax return which is 2019.
Prior year returns cannot be e-filed with TurboTax. To file your 2018 return with TurboTax you will need to use the 2018 Download software (or go to the IRS website for prior year forms). To purchase the software from TurboTax go to the following website.
https://turbotax.intuit.com/personal-taxes/past-years-products/
