Prior-year returns cannot be e-filed; they must be printed and filed. I'm sharing a TurboTax FAQ that will explain how you'd now complete your 2017 return that you started in TurboTax Online. You'll need to purchase the 2017 TurboTax Desktop (CD/download) software in order to file.
https://ttlc.intuit.com/replies/4186197
*** I am NOT a tax expert. I am a seasoned TurboTax user, and volunteer to provide assistance to TT users. Nothing I post is to be considered TAX ADVICE; I bear no legal liability for responses.***