Going to a traditional IRA, the taxable amount of the distribution is zero. The payer *might* have marked the Taxable amount not determined box and left box 2a blank because they do not know if the deceased had any after-tax contributions in the plan. However, that seems improbable since after-tax money in the plan must be kept in a separate sub-account, so they would know if there was any or not. Even if there was any after-tax money in the account, since they didn't tell you in box 5 than any portion of the distribution was after-tax money, I don't know how you would be able to determine how much was after tax. You'd likely have to treat it all as pre-tax money anyway.
Make sure that you have selected the code 4 in one of TurboTax's box 7 drop-down boxes and code G in the other.
TurboTax will not treat this distribution as taxable unless you indicate that it was rolled to a designated Roth account (generally not an option with a distribution from an inherited account) or to a Roth IRA. Since it was apparently not rolled over to a Roth account of any kind, the result should be that the box 1 amount will be included on Form 1040 line 16a or Form 1040A line 12a but the entire amount will be excluded from the amount on line 16b or 12b. The word ROLLOVER should appear next to the line. If this is not the result you are seeing, delete and reenter the Form 1099-R.
Note that TurboTax's summary screen show the total of both taxable and nontaxable income. Do not try to interpret the summary screen, look at your Form 1040 or 1040A.
