401K and Roth conversion

In order to populate from 8606, when you enter the traditional IRA contribution in the deductions section (separately from the conversion reported on the 1099-R in the income section), you should answer "Yes, make part of my IRA contribution nondeductible" in the interview.
 

Here is the full list of steps to report a backdoor Roth in TurboTax Online, from How do I enter a backdoor Roth IRA conversion?:

"Step 1:  Enter the Non-Deductible Contribution to a Traditional IRA

1. Go to Deductions & Credits
2. On the Your tax break screen, locate and select the Retirement and Investments section, then select Start or Revisit next to Traditional IRA and Roth IRA.
3. On the Traditional IRA and Roth IRA screen, check the Traditional IRA box and select Continue.
4. On the Is This a Repayment of a Retirement Distribution? screen, select No.
5. On the Tell Us How Much You Contributed screen, enter the amount contributed and select Continue.
6. On the Did You Change Your Mind? screen, select No, then answer the questions on the following screens.
7. When you reach the Choose Not to Deduct IRA Contributions screen:
8. Select Yes, make part of my IRA contribution nondeductible.
9. Enter the amount you contributed.
10. If none of your contributions can be deducted, you won't see the option to make part of it nondeductible. Instead, you will see the message Income Too High To Deduct an IRA Contribution, and the full amount of your contribution will automatically be nondeductible.
11. Select Continue.
12. On the Your IRA Deduction Summary screen, review the info and select Continue.

Step 2:  Enter the Conversion from a Traditional IRA to a Roth IRA

1. Go to Wages & Income.
2. On the Your income and expenses screen, locate and select the Retirement Plans and Social Security section, then select Start or Revisit next to IRA, 401(k), Pension Plan Withdrawals (1099-R).
3. Answer Yes to Did you get a 1099-R in 2024?, then Continue.

• If you land on the screen Review your 1099-R info select Add Another 1099-R.

1. Select how you want to enter your 1099-R (import or choose Change how I enter my form to either upload it or type it in yourself) and follow the instructions.
2. Answer the questions on the following screens, until you reach Tell us if you moved the money through a rollover or conversion.
3. Select I converted some or all of it to a Roth IRA and Continue.
4. Next, choose Yes, all of this money was converted to a Roth IRA and Continue..
5. Continue answering questions until you come to the screen Review your 1099-R info."