The only option when I went through the interview was to enter a non-deductible portion for the year 2024.
It also didn't ask for my spouse whose entire 2024 contribution was non-deductible, and who also has a past basis in non-deductible contributions from before 2024.
Has the interview process been altered or fixed? This will be a recurring problem with TurboTax if you're not allowed to bring in past contributions, the 8606 amounts will always be wrong.
