If your Excel sheet is not allowing you to use the undo function, here are some tips to troubleshoot:

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1. Check Excel’s AutoRecover feature is set up
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2. Check to be sure the workbook has not been protected
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3. Run a scan of the workbook for any viruses or malware
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4. Update to the most current version of Excel
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5. Check for conflicting add-ins that may be disabling the undo