If I understand correctly, when you open the file in Excel you receive an error in Excel itself? It would probably be most helpful if you can attach a screenshot of the complete error you are seeing.
The DSS pivot table chart download will download in xlsx format, which is generally accepted on Excel 2007 and later. There may be some possibility that your version of Excel is older and expects xls format. Can you verify the version of Excel you are using? If you might be using quite an old version, you could try using google sheets to import the pivot table xlsx file instead.