You cannot file prior year returns with the Online version. The Online version is only to prepare the current years tax return (2019). To use TurboTax to file your 2018 return you would need to download their 2018 Desktop software. You also need to mail in your 2018 return, it cannot be e-filed. You can purchase the 2018 Desktop software from TurboTax at the following website.
https://turbotax.intuit.com/personal-taxes/past-years-products/
