TurboTax only asks you to separately determine and enter the taxable amount if the distribution from an inherited IRA, reported with code 4 in box 7. If the deceased had basis in nondeductible traditional IRA contributions, you must prepare outside of TurboTax a Form 8606 to calculate the taxable amount paid to you from the deceased's IRAs of which you are the beneficiary. You'll then enter the taxable amount from line 15 of that Form 8606.
Box 2a from any Form 1099-R reporting regular distributions (including distributions of RMDs) from an IRA is always to be ignored and box 2b Taxable amount not determined is always to be marked, per the IRS instructions for Form 1099-R.
