A #REF! error occurs when a cell reference to a formula or range name is invalid. This usually means that a formula/range name is missing or has been deleted.
#REF! errors can occur if a user deletes or cuts a cell out of the spreadsheet which is used as a reference for a formula or calculation.
If a #REF! is found in the spreadsheet due to user modifications, you will likely need to start a new project with a clean spreadsheet.