Is box 2b Taxable amount not determined marked on the form provided by the pension plan?
If past Forms 1099-R from the pension plan had the same amount in box 2a as is in box 1, it seems erroneous that the pension plan would leave box 2a blank on the 2023 Form 1099-R. If box 2a is erroneously blank, answering TurboTax's questions exactly as asked will lead to an erroneous result ($0 taxable).
If past Forms 1099-R had box 2a blank and box 2b Taxable amount not determined marked, it seems that your father probably answered that an different amount than the amount shown in box 2a was taxable, then used the Simplified Method where he entered the annuity start date (it seems that any date will work), a plan cost of $0 and 12 month of 2023 payments, resulting in the entire distribution being taxable.
If TurboTax was used for the 2023 tax return, examine TurboTax's 1099-R form for this distribution and see if anything is present in box 2a or if there is anything present on the Simplified Method Worksheet at the bottom of this form.
