One possibility is that your tax2015 file was saved with a password. You cannot transfer a password-protected file to online. You need to remove the password fist. But you cannot remove the password if you are on Windows XP, as the password is mandatory in Windows XP. If you are on Windows XP, you need to install your old desktop TurboTax 2015 software on a Windows 7 machine and download the state to complete the installation. Then open the tax2015 file which has a password with the desktop TurboTax 2015 you installed on Windows 7. Once opened, select File > Password and you will see a button "Remove Password". Click that button and save the file, then use it for transferring to online. This issue was discussed in this thread:
https://ttlc.intuit.com/questions/3867262-i-m-getting-the-error-the-selected-file-is-not-a-previous-...
