If your Form 1040 line 4a and 4b are both showing $100,000 then the full amount of the Form 1099-R distribution is being included in your income. So, regardless of the summary page in TurboTax saying the amount is not taxable, it is being taxed on your actual tax return.
Since the Traditional IRA that is being converted did not have any non-deductible contributions, then it is expected that the entire distribution would be taxable. The only reason it would not be fully taxable would be if you had more than one Traditional IRA account and the other account(s) had non-deductible contributions. The 'taxable amount not determined' check box indicates that the situation about whether you had any non-deductible contributions to consider was not known to the IRA custodian.
Box 4 on your Form 1099-R indicates that no Federal taxes were withheld from the distribution to help cover the income tax.
@KelleyR
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