I guess when I efiled in 2015 I must not have saved it on my computer? I don't think I understand that last comment about "if you have the 2015 tax data which has a file extension of .tax2015".....?
Only the tax data file from tax year 2015 can be used to transfer the 2015 tax data to the 2016 tax return. If you saved the 2015 online return as a PDF and the tax data file, then tax data file, .tax2015, should be stored on your computer.
To access your current or prior year online tax returns sign onto the TurboTax website with the userID you used to create the account - https://myturbotax.intuit.com/
Scroll down to the bottom of the screen and on the section Your tax returns & documents click on Show. Click on 2015 and Click on Download .tax file
If the system is showing that you need to pay for the account when trying to download the .tax file, then follow this procedure -
See this TurboTax support FAQ for when access to prior year online tax returns expire - https://ttlc.intuit.com/community/prior-year-return/help/when-does-access-to-my-prior-year-turbotax-...
To access a prior year online tax return and you did not use TurboTax for the 2017 tax return or you did not pay for one of the 2017 online editions of either Plus, Deluxe, Premier or Self-Employed, you will need to start a 2018 Free online tax return and then Upgrade and pay for the Plus edition ($19.99). You do not have to file the 2018 tax return only pay for the upgrade.
Go to this TurboTax website for the 2018 online editions - https://turbotax.intuit.com/personal-taxes/online/
You have to sign onto the 2018 account using the exact same User ID you used to create the prior year online tax return. Enter your personal information in the 2018 Free edition. On the left side of the screen click on Upgrade. Scroll down and select PLUS More Help & Tools. Click on Upgrade Now (if Plus is grayed out then you are already using Plus and only need to pay for the upgrade)
To pay the TurboTax online account fees by credit card so you can access your prior year online tax returns, click on Tax Tools on the left side of the online program screen. Then click on Print Center. Then click on Print, save or preview this year's return. On the next page, to pay by credit card, click Pay Now. On the next screen just click on the Continue button. The next screen will show a review of your Fees, click on Continue. Complete the next screen to pay by credit card and click Continue.