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 must mail them in (make sure your sign the return). To file your 2017 return with TurboTax you will need to use the 2017 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/2017
