I have 2023 and 2024 IRA contributions on single 2024 1099-R ($13,500), but getting excess contribution error. How do I do backdoor Roth when both years are on the form?

You should have reported the nondeductible Traditional IRA contribution for 2023 on your 2023 return and have a 2023 Form 8606 with the basis on  line 14. This basis has to be entered on your 2024 return.

You will enter the nondeductible Traditional IRA contribution for 2024 on your 2024 return:

1. Login to your TurboTax Account 
2. Click on "Search" on the top right and type “IRA contributions” 
3. Click on “Jump to IRA contributions"
4. Select “traditional IRA”
5. Answer “No” to “Is This a Repayment of a Retirement Distribution?”
6. Enter the amount you contributed
7. Answer “No” to the recharacterized question on the “Did You Change Your Mind?” screen
8. Answer the next questions until you get to “Any Nondeductible Contributions to Your IRA?” and select “Yes” if you had a nondeductible contribution before this tax year.
9. Enter your basis in the Traditional IRA from your 2023 Form 8606 line 14 (if you had a basis in the prior year)
10. On the “Choose Not to Deduct IRA Contributions” screen choose “Yes, make part of my IRA contribution nondeductible” and enter the amount (if you have a retirement plan at work and are over the income limit it will be nondeductible automatically and you only get a warning and then a screen saying $0 is deductible).

To enter the Form 1099-R conversion in TurboTax Online: 

1. Click on "Search" on the top right and type “1099-R”  
2. Click on “Jump to 1099-R”
3. Click "Continue" and enter the information from your 1099-R
4. Answer questions until you get to “Tell us if you moved the money through a rollover or conversion" screen and choose “I converted some or all of it to a Roth IRA”
5. On the "Here's your 1099-R info" screen click "Continue"
6. Answer "Yes" to "Any nondeductible Contributions to your IRA?" if you had any nondeductible contributions in prior years.
7. Answer the questions about the basis from line 14 of your 2023 Form 8606 and the value of all traditional, SEP, and SIMPLE IRAs

To enter the Form 1099-R conversion in TurboTax Desktop: 

1. Click on "Search" on the top right and type “1099-R”  
2. Click on “Jump to 1099-R”
3.  Enter the information from your 1099-R
4. Answer questions until you get to “What Did You Do With The Money” and choose “I moved it to another retirement account”
5. Then choose “I did a combination of rolling over, converting, or cashing out money.” and enter the amount next to "Amount converted to a Roth IRA account"
6. On the "Your 1099-R Entries" screen click "continue"
7. Answer "yes" to "Any nondeductible Contribution to your IRA?" if you had any nondeductible contributions in prior years.
8. Answer the questions about the basis from line 14 of your 2023 Form 8606 and the value of all traditional, SEP, and SIMPLE IRAs

Please see How do I enter a backdoor Roth IRA conversion?  for additional information.