I am trying to file my 2015 tax return. It

[rjs]

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.

[VolvoGirl]

If you want to do 2015 for free you can get the blank forms from the IRS and filll them out by hand.  Here are some blank forms & instructions for 2015
1040 https://www.irs.gov/pub/irs-prior/f1040--2015.pdf     Instructions https://www.irs.gov/pub/irs-prior/i1040gi--2015.pdf

1040A https://www.irs.gov/pub/irs-prior/f1040a--2015.pdf    Instructions https://www.irs.gov/pub/irs-prior/i1040a--2015.pdf

1040EZ https://www.irs.gov/pub/irs-prior/f1040ez--2015.pdf    Instructions https://www.irs.gov/pub/irs-prior/i1040ez--2015.pdf

2015 Tax Tables https://www.irs.gov/pub/irs-prior/i1040tt--2015.pdf
Publication 17 Tax Guide for 2015 https://www.irs.gov/pub/irs-prior/p17--2015.pdf

THEN FOR 2016:
You can still use your Online account to do this year's 2016 return but you will need to clear out your entries and start over for 2016.  Or abandon this online account and start a new one.
https://ttlc.intuit.com/questions/2586254-using-clear-start-over-in-turbotax-online