We suggest that you first contact CRA and verify the details of this reassessment with them. Then, if you think it's a calculation error from the software, we suggest you contact Product support by phone to verify if the modification applied by CRA is subject to a claim under the TurboTax 100% Accurate Calculations Guarantee.
Visit this link before contacting support. You'll find all the information you need to be prepared for this process.
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