you can either handle this yourself if you didn't buy audit defense at the time or you can still notify them but there will be a fee.
The Audit Defense service is provided by TaxResources, Inc., also called TaxAudit.com, in partnership with TurboTax. If you paid for Audit Defense and received a notice, call TaxResources, Inc. at 877-829-9695, or report your notice on their website at http://intuit.taxaudit.com/. Do not contact the tax agency until you have spoken to TaxResources. They may contact them on your behalf.
as to Turbotax assurances - if it made an error it's responsible for your penalties and interest but not the taxes you would have owed anyway. File a claim. It's also possible this is an IRS error. For example sending in an estimated tax payment by mail that is missing identifying info. the IRS keeps the money but the lack of info prevents it from being credited to your account. You should get a letter of explanation or can call them. Also, the difference should show up in your transcript for that year. You can access it on the IRS website. Creation of an account with ID.me is necessary unless you've done this previously.
https://sa.www4.irs.gov/sso/?resumePath=%2Fas%2Flob9XVrgQQ%2Fresume%2Fas%2Fauthorization.ping&allowI...
