A common reason for Social Security to not be included as taxable income when it should be is an inaccurate answer to the question asking about being a US citizen living in certain foreign countries. If you mistakenly answered 'yes', then none of your Social Security would be taxed.
If your CP11 notice included any penalties and interest and your entries into TurboTax were accurate, then you can submit a claim through the TurboTax guarantee. See the following TurboTax help article for details:
How do I submit a claim under the TurboTax 100% Accurate Calculation Guarantee or 100% Accurate, Exp...
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