Turbo Tax online is only good for 2016 returns now. You have to buy and download and install the prior year Desktop program to do an older return. There is no penalty if you are getting a refund. You have to mail in past year returns. You can buy the prior year programs here,
https://turbotax.intuit.com/personal-taxes/past-years-products
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
