It is not possible to file a 2015 tax return using DIY software. You would need to use a tax professional in your area to assist in completing and filing the 2015 tax return.
If you expect a refund for tax year 2015 it will not be paid. It is beyond the three year limit from the due date of the tax return for a refund.
If you are a US citizen or legal resident you are required to report your worldwide income on a federal tax return each tax year.
TurboTax supports tax years 2022 thru 2019.
TurboTax software is no longer available for 2015. And any refund you could have received for a 2015 return has been forfeited. If you owe tax due, however, you still need to pay with interest and penalties. You could seek local paid tax help or you can use the paper forms and mail in the return with your check or money order.
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
You say that you have been out of the U.S. for several years and have not filed tax returns. Are you aware that if you are a U.S. citizen you are required to file returns for your world-wide income even if you were not in the U.S.?
Each tax year has to be filed separately using the forms for the specific tax year. They cannot be combined in any way--do not even put them in the same envelope when you mail them. Software for past years is available back to 2019. Any earlier years can only be prepared on paper forms by hand--and those refunds have been forfeited now.
The current online program can only be used for a 2022 return--no other year. And if you did not file a 2021 return you will have trouble e-filing 2022. When it asks for your 2021 AGI try using zero. If zero does not work then you have to print, sign and mail 2022
TurboTax has software for 2019 onward. Software is no longer available from TurboTax for earlier years, nor would it be supported even if you find it on eBay, etc. Updates are not available. It is also too late to receive a tax refund for a tax year earlier than 2019---those refunds have been forfeited. BUT if you owe tax due you still have to pay with penalties and interest. There might be other software providers that have software for those years, but you will not receive refunds for the returns even if the forms show a refund.
It might be better for you to seek local paid tax help in a situation when you are several years behind in filing your tax returns. Do you even have all of the documents you will need to enter and show your income—like your W-2’s and any 1099’s?