Yes, you can file your 2019 Online, but you would need the 2018 Download software for your 2018 return. You cannot file prior year returns with the Online version. The Online version is only to file the current years tax return (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/
