Read the notice carefully and review your actual return ... did you make the payments listed ? Did you forget to list a payment ?
We changed your return because we found a difference between the
amount of estimated tax payments on your tax return and the amount we posted to your account. You have a balance due because of these changes. The notice may also cover other changes we made while processing your return.
https://www.irs.gov/individuals/understanding-your-cp23-notice
