You can start a 2017 tax return at any time. You will have to purchase, download and install on a personal computer one of the 2017 desktop editions from this website - https://turbotax.intuit.com/personal-taxes/past-years-products/
A 2017 tax return can only be printed and mailed, it cannot be e-filed.
The TurboTax online editions are only for tax year 2018. You can e-file the 2018 tax return at any time up through October 15, 2019. After that date the 2018 tax return can only be printed and mailed. The TurboTax online editions will be shutting down after October 15 to make way for the 2019 online editions.
Online TurboTax can only prepare a 2018 return. To prepare a 2017 return, you would need to purchase 2017 desktop software.
The 2017 return cannot be efiled. It has to be printed, signed, dated, and mailed.
If you efile the 2018 return, it will ask you for the 2017 AGI for verification. Since you did not file a 2017 return before November 2018, you would use a 2017 AGI of 0.
2017 can only be done using the desktop software installed on a Windows or Mac computer. 2017 can only be paper filed and mailed.
You can purchase the 2017 software here:
You should do your 2017 return first. Some information from your 2017 return is used on your 2018 return. You can always go back and add or change information, but it is much easier if you do 2017 first, then Turbo Tax will carry necessary information forward.