TurboTax automatically transfers AGIs from year to year for all customers. If you don't see your 2015 AGI in this account, that means you may have used a different account in 2015.
Try logging into the account you used last year-- your 2015 AGI will be in your tax timeline. If you can't find your previous log in or can't do an account recovery, you can go to IRS.gov to obtain your AGI from last year.