TurboTax guarantees the calculation made on the tax return. If there is an error on any calculation TurboTax will pay any penalties and interest, but additional taxes will be paid by the taxpayer. User input errors or omissions are not covered by the accuracy guarantee.
The IRS Letter 4800C proposes changes to the tax return that was filed. What are the changes and do you agree or disagree with the proposed changes?
See this TurboTax support FAQ for submitting an accuracy guarantee claim - https://ttlc.intuit.com/turbotax-support/en-us/help-article/intuit-account-billing/submit-claim-turbotax-100-accurate-calculation-100/L88SYklBL_US_en_US?uid=m5e8sll5