You cannot use the Online version to file a prior year return, the Online version is only to prepare a 2019 return. To file a 2018 return with TurboTax you would need to download the 2018 tax software & install it on you computer. Prior year returns cannot be e-filed with TurboTax, you would need to mail your returns in.
You can download the 2018 TurboTax software at the following website
https://turbotax.intuit.com/personal-taxes/past-years-products/
