It's not too late to file your 2018 tax return. You cannot file a previous years return with the Online version, the Online version is only to file the current years tax return.
Prior year returns cannot be e-filed with TurboTax. You 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/
