I am not expert on this. I will say that in my very limited experience with the IRS & penalties, they usually are correct. Perhaps the following links can help.
https://www.irs.gov/individuals/understanding-your-cp23-notice
https://www.irs.gov/payments/failure-to-pay-penalty
The above sites also have links to related sites.
If this does not meet your needs, and others do not answer more specifically, you may want to provide additional information: when you filed your 2021 return, total tax due on your 2021 return, date of the CP23 notice and any explanation of their computation of the penalty.
You may be seeing a penalty, plus interest charges, or perhaps multiple penalty charges if you did not timely file your return.
Lastly, consider paying the penalty soon to stop the accrual of interest and then, if appropriate, contest the charges.
Good Luck
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