Sorry---you cannot use the online program to file a 2015 tax return. The online program is only for a 2019 tax return.
In fact, software is no longer available for a 2015 tax return. If you want to file the return you can use the paper forms and mail them in. You will not receive a tax refund for a 2015 tax return. It is too late.
https://www.irs.gov/filing/where-to-file-paper-tax-returns-with-or-without-a-payment
2015
Form 1040 https://www.irs.gov/pub/irs-prior/f1040--2015.pdf
Form1040 Instructions https://www.irs.gov/pub/irs-prior/i1040gi--2015.pdf
Form1040A https://www.irs.gov/pub/irs-prior/f1040a--2015.pdf
Form 1040A Instructions https://www.irs.gov/pub/irs-prior/i1040a--2015.pdf
Form 1040EZ https://www.irs.gov/pub/irs-prior/f1040ez--2015.pdf
Form 1040EZ Instructions https://www.irs.gov/pub/irs-prior/i1040ez--2015.pdf
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