First of all check if the application have permissions to write to the folder?
If you have permissions, check this link;
http://www.made4dotnet.com/Default.aspx?tabid=141&aid=15[
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If that didnt solve maybe you are trying to save more than 256 columns of data. XLS files only support 256 columns of data.You may want to;
Change the data matrix such that it had fewer than 256 columns of data,
or
write to XLSX file. The new XLSX file format supports many more columns of data
Good luck