When did you do the conversion and how/when did you pay any estimated taxes?
In general, taxes are supposed to be paid evenly over the year. If you did a conversion of (let's say) $40,000 and your taxes are estimated to be $10,000, you would be expected to pay quarterly payments of $2500 by April 15, 2024, June 15, September 15, and January 15,, 2025. These deadlines apply even if you did the conversion at the end of the year. So you can be considered to be "under-paid" or "under-withheld" if you did not make the estimated payments, even if you are prepared to pay in full when you file.
With a late conversion, you need to make the estimated payment, and then you need to file form 2210 using the "annualized penalty method" to show the IRS that even though your payments were not even over the year, each quarterly payment was appropriate for that quarter's income.
