I was unable to file my 2017 return due to an illness. Can I file it now on Turbo tax?

Online TurboTax can only prepare a 2019 return at this point in time.   For a 2017 return, if you want to use TurboTax, you'll need to use 2017 desktop software (CD/download).  And the 2017 return cannot be efiled; it has to be filed by mail.   I'll also tell you another option at the end of this post if you want to prepare it manually with forms and instructions from the IRS.

You can purchase 2017 desktop software as a download at this page and install it on your computer:

https://turbotax.intuit.com/personal-taxes/past-years-products/2017

Minimum system requirements for Windows version:
https://ttlc.intuit.com/questions/1901571-minimum-system-requirements-for-turbotax-windows-software

Minimum system requirements for Mac version:
https://ttlc.intuit.com/questions/1899173-minimum-system-requirements-for-turbotax-mac-software

The 2017 return cannot be efiled.  It will need to be printed, signed, dated, and mailed.  It's a good idea to mail by a method that has tracking, such as certified mail, priority mail, etc.

If you have both Federal and state returns, you need to mail each one separately to its respective tax agency. 

If you are due a Federal refund, in "normal times" it would take about 6 weeks for the IRS to process a mailed return and deliver the refund. But these are not normal times, so expect delays. Due to the Coronavirus situation and limited personnel, the IRS is currently not actively processing paper returns.

Or did you already start a 2017 return online prior to the 2017 online shutdown, and want to finish that one without starting over?  If so, you can download the *.tax2017 data file from your account and finish it in the 2017 desktop software.    If you already paid for your 2017 product previously, TurboTax Support should furnish you the software.

OR another option is to download the 2017 forms and instructions for free from the IRS website and fill them out manually.

https://www.irs.gov/forms-pubs/prior-year