If you are getting the Data source reference is not valid while trying to create a pivot table in Microsoft Excel, see this post for the fix.

If Excel is not saving formatting, this article will help you know the common causes of this problem and the ways to fix the issue.

How to Recover a Microsoft Excel File That Is Not in a Recognizable Format. If Microsoft Excel fails to recognize a file in the XLSX or XLS format, the data contained in the file could be corrupted.