Box 6 is informational only. It has nothing to do with determining the taxable amount of the distribution.
If the entire amount was used for qualified education expenses of the designated beneficiary of the 529 account, as appears to be the case here, the distribution is nontaxable. If TurboTax is determining some amount to be taxable, you haven't properly indicated to TurboTax the qualified education expenses of the designated beneficiary. Make sure that you have clicked the Continue button on the Form 1099-Q Summary page.
