You cannot file a prior year return Online, the Online version is only for the current years tax return (2018 now). To file your 2017 return with TurboTax, you would need to purchase the 2017 Desktop software & install the software on your computer. You cannot e-file a prior year return, it must be mailed.
You can purchase the 2017 TurboTax Desktop software at the following website
https://turbotax.intuit.com/personal-taxes/past-years-products/
