"Taxable upon conversion" simply means a conversion that is subject to inclusion in AGI. It has nothing to do with the timing of the payment of any tax liability. Similarly, "Nontaxable upon conversion" means a conversion that is not subject to inclusion in AGI because it is a conversion of after-tax basis.
The conversions that you did are income in the first quarter of 2024. Since you made no estimated tax payments, if your tax withholding for 2024 was not sufficient to meet any of the safe-harbors for an underpayment of taxes, you are subject to an underpayment penalty. Your tax withholding and income are by default treated as paid evenly throughout the year and underpayment is determined on a quarterly basis. Depending on how much tax withholding you actually had during each of the tax quarters, you might benefit from annualizing income and tax withholding, but given that you probably received a large portion of your taxable income in Q1, annualizing might actually increase your underpayment penalty.
Make sure that you've told TurboTax your 2023 tax liability so that TurboTax can take into account the safe-harbor that is based on that. If you used TurboTax for 2023, that information should have transferred into 2024 TurboTax when you began your 2024 tax return.
Regarding underpayment of taxes: https://www.irs.gov/taxtopics/tc306
