If you owed tax due for 2015 why do you think you would now receive a refund if you file 2015? There is no statute of limitations for the IRS to get paid for tax due---you owe it until you pay it. You have three years to receive a refund, but you are not given a pass if you owe---you still owe for 2015 and they are not going to give your money back after offsetting that money from your 2020 refund. Or am I misunderstanding what you are asking?
You can try filing the 2015 return. You will not be able to use TT software---that software is no longer available. You can prepare the return yourself using the instructions and paper forms.
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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