You should have reported the nondeductible contribution on your 2023 return and should have a 2023 Form 8606 with a basis on line 14. This will be entered during the interview when TurboTax asks "Any nondeductible Contributions to your IRA?". If you do not get this question then please see Why am I unable to report my 2023 for 8606 basis?
If you do not have a 2023 Form 8606, then you will need to file it with the IRS. Please see If your conversion contains contributions made in 2024 for 2023.
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