Download your tax return transcript for your 2019 tax return to see where the IRS Automated UnderReporter system might have flagged a difference between what you reported on your tax return and what the IRS system calculated: https://www.irs.gov/individuals/get-transcript
There is certainly no penalty on a Roth conversion, but the conversion does add to your AGI. The standard deduction (or itemized deductions if you were to itemize) is then subtracted to AGI to get taxable income on which income tax is calculated. If there had been a transaction that was subject to a 10% early-distribution penalty, the penalty would be added to the income tax liability. For example, if before 5 years from the beginning of 2019 you subsequently took a distribution of an amount attributable to a taxable conversion to Roth, that distribution would be subject to a recapture of the 10% early-distribution penalty even though it would not be subject to income tax.
