You can still file a 2015 return, but you cannot prepare it in Online TurboTax, which is now for 2018 only. You have to use the 2015 desktop software (CD/download.)
Also, if you are due a 2015 Federal tax refund, while you can still file the return, the deadline to claim a 2015 Federal refund was April 15, 2019, unless you originally filed for an extension back in 2015.
You can purchase 2015 desktop software as a download at this page and install it on your computer:
https://turbotax.intuit.com/personal-taxes/past-years-products/2015/
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
Or did you already start a 2015 return online prior to the 2015 online shutdown, and want to finish that without starting over? If so, you can download the *.tax2015 data file from your account and finish it in the desktop software. If you already paid for your product previously, TurboTax Support should furnish you the software.
The 2015 return cannot be efiled. It will need to be printed, signed, dated, and mailed. It is recommended to mail by a method that has tracking such as certified return receipt, 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, it takes about 6 weeks for the IRS to process a mailed return.
