You cannot efile any tax returns other than 2019 using TurboTax. All prior year returns must be printed and mailed. TurboTax only provides prior year tax preparation programs for the 3 years prior to the current year so 2015 is not available from TurboTax any more.
You have 3 years from the tax returns original filing deadline to file a return for a refund. You should still file older returns if you have not previously filed them but they are not eligible for a refund. You must, however, pay any taxes due on a prior year tax return regardless of how old the tax return is.
