Did you use Online TurboTax in 2015? If you used the desktop software that year (CD/download), it won't be in an online account.
If you did use Online Turbotax, here's how to troubleshoot multiple accounts (User IDs). To get a list of your User IDs and recover account access, you can use the tool at the link below. Try using your phone number first. If you try using your email address, and you have more than 1 email address, you should run the tool on each one.
NOTE: Before running the account recovery tool below, log out of all Intuit accounts including this user forum, or you might end up in a loop. Then clear your browser Internet cache, close your browser, then reopen it, and go to the link below. You may wish to copy this link so you can paste it into the new browser session.
If still no luck after running that on each email address you have, then run the tool yet again, but this time choose the small blue link that says "Try something else", and it will look you up by SSN and other parameters.