Online TurboTax is only for 2016 returns. A 2015 return can only be prepared in desktop software (CD/download) at this point in time. And it cannot be efiled; it has to be printed, signed, and mailed.
If you started a 2015 online return prior to the October shutdown, then you can download the *.tax2015 data file from your 2015 Tax Timeline and continue it in purchased 2015 desktop software. If you already paid for an Online return, TurboTax Support should furnish you a download.
You can purchase 2015 software from TurboTax at the link below, or you may be able to find it at a discount at a reputable authorized retailer such as Amazon, etc.
https://turbotax.intuit.com/personal-taxes/past-years-products/
If you already paid for a 2015 online product, here's how to reach TurboTax Support during business hours:
FAQ: How do I contact TurboTax?
https://ttlc.intuit.com/questions/3401489
