It's normal for "taxable amount not determined" to be checked for an IRA distribution. The IRS instructions for Form 1099-R say to check it.
On the 1099-R the taxable amount in box 2a should be the same as the gross distribution in box 1. Did you enter both amounts that way?
Where and how does TurboTax say the distribution is nontaxable?
Look at your Form 1040. What are the exact amounts on lines 4a and 4b. If this is your only IRA distribution, line 4a should be blank and line 4b should be the amount from boxes 1 and 2a on the 1099-R. The amount on 1040 line 4b is taxable. Are any of the boxes checked on line 4c? If so, which one?