You cannot file a 2018 return with the Online version. The Online version is only for the current tax season (2019). You would need the download TurboTax software for 2018. Prior year returns must be mailed. You can purchase the 2018 download software from TurboTax, at the following website, or (check the IRS website for 2018 forms)
https://turbotax.intuit.com/personal-taxes/past-years-products/
