Assuming that you have no other IRA accounts and that the traditional IRA contribution for 2023 that you enter into 2023 TurboTax was nondeductible, TurboTax will prepare 2023 Form 8606 showing the nondeductible contribution on line 1. TurboTax will then apply that basis to the Roth conversion performed in 2023, resulting in $0 of taxable income shown on line 18. The amount on line 18 gets included on Form 1040 line 4b, so with the amount on line 18 of Form 8606 being zero, nothing is added to line 4b. If no Form 8606 is present, you have failed to enter a nondeductible traditional IRA contribution.
