The response by TurboTax suggests that you have indicated that this is a distribution from an inherited IRA. A distribution from an inherited IRA would have code 4 in box 7 with the IRA/SEP/SIMPLE box marked.
Distributions from an inherited IRA do not go on your Form 8606. The calculation must be done on a separate Form 8606 outside of TurboTax. TurboTax does not support generating Forms 8606 for distributions from inherited IRAs.
If the distribution is not from an inherited IRA and the Form 1099-R has code 1, 2 or 7 in box 7 with the IRA/SEP/SIMPLE box marked, I suggest deleting the Form 1099-R from TurboTax and reentering it, making sure to not indicate that the distribution is from an inherited IRA.
