You cannot use TurboTax Online to file for 2015. TurboTax Online is only for the current tax year, which is now 2016. To file for 2015 you have to use the CD or download TurboTax software for 2015. You can purchase a download at the following link.
http://turbotax.intuit.com/personal-taxes/past-years-products.jsp
You might get a better price from a reputable online seller that still has TurboTax for 2015 in stock.
TurboTax does not allow returns for past years to be e-filed. You will have to print your 2015 tax return and file it by mail.
See this TurboTax support FAQ on completing a prior year tax return started with the online editions - https://ttlc.intuit.com/questions/2559403-how-do-i-finish-a-prior-year-return-that-i-started-in-turb...
If you purchased one of the 2015 online editions of Plus, Deluxe, Premier or Home and Business, then contact TurboTax support so they can push a 2015 desktop installation file to your download account.
A tax year 2015 return can only be printed and mailed, it cannot be e-filed.
See this TurboTax support FAQ for a contact link and hours of operation - https://ttlc.intuit.com/questions/1899263-what-is-the-turbotax-phone-number
To contact TurboTax customer service/support use their contact website during business hours.
Submit your question/problem during business hours and then use the option for Call (a phone number will be provided for Call) to speak directly with a TurboTax agent -
https://support.turbotax.intuit.com/contact/
Note: The Call option will not be available when TurboTax Support is closed.