You are correct that the Online TurboTax program can only be used to create 2018 tax returns.
If you need to file a 2017 tax return (and you never started one), you would need to purchase a download/CD version of the software and file the return from. See this link: TurboTax
Previous Years Products
If you started a 2017 return online, but did not finish it/file it, see the instructions in this link below:
https://ttlc.intuit.com/replies/4186197
If you need to amend your 2017 tax return in order to include the 1099C form, you can use the steps below:
https://ttlc.intuit.com/replies/6603004
