You cannot open XLS files with OpenXml. You must save your Excel file as an XLSX file using Microsoft Excel 2007 or newer. (Assuming you are using the correct file extension here...)
Files with XLS extension are in a proprietary binary format which cannot be processed with OpenXML.
So you have two options:
- Convert your XLS files to XLSX manually before processing them.
- Find a library or API to process XLS files.