For your Excel template file, if you could open your XLS file into for example MS Excel 2003 and click on the SaveAs, the "Save as type" would be something like "Microsoft Excel 5.0/95 Workbook" file type for so. I am afraid, Aspose.Cells does not support to read/write BIFF7 or older file formats as it only supports BIFF8 and greater file formats (Excel 97 - Excel 2010, Excel 2013), so you are getting this exception.
We can only suggest a workaround for you as we cannot support the older file formats in our product. You may try the following steps to work with these files for Aspose.Cells product, so that Aspose.Cells could open/process the file fine, there is no alternative I am afraid:
Open your template file into MS Excel (e.g..,Ms Excel 2003 or 2007).
Save the file As "Microsoft Office Excel Workbook".
Now you can use Aspose.Cells to open/manipulate, render or convert the file for your requirements, it will work fine.