Solution 7: Open MS Excel in Safe Mode Solving the “ Excel cannot open because the file format is not valid ” problem can sometimes be as simple as booting your computer into Safe Mode . This is ...

With the Excel extension is not valid error, try starting the app in Safe Mode, repair the workbook or open the file in another program.

Please ensure the .xls file is not opened in other workbooks, add-ins, or apps. After coming to that conclusion, simply attempt to open the file again in Excel to see if the problem no longer exists.