The IRS sends CP23 to notify you that due to a discrepancy between estimated tax credits claimed on your return and the amounts the IRS posted to your tax account. Did you indeed make estimated tax payments (other than normal withholdings)?
Who made a mistake? If you made a user error and did not enter income or entered something incorrectly. the accuracy guarantee does not help with that.
(what the letters from the IRS mean)
I GOT A LETTER FROM THE IRS
I AM BEING AUDITED
If you purchased Audit Defense:
TurboTax has a special phone number for help with IRS/state tax letters, which will be easier than going through regular Customer Support. At the page below, click on the blue button that says "Get Help from TurboTax Support". Then it will ask you what tax year is your letter. Then it will ask you what the letter concerns. Then it will show you the phone number during posted business hours.