We don't know your setup. First, set the path explicitly (use absolute path) to see you have access rights. Then use relative, but use
Try using Path.GetFullPath ("~/Data/File.xls") to see where it maps
Server.MapPath("~/Data/File.xls")
- this could (maybe) resolve your problem - THIS ASSUMES WEB APP although you didn't specify it :)
If this helps please take time to accept the solution. Thank you.