How to fix error - Could not find a parto of the path

Replace

       using (SpreadsheetDocument sd = SpreadsheetDocument.Open(FileUpload1.PostedFile.FileName, false))
            {
     
            }

with

       using (SpreadsheetDocument sd = SpreadsheetDocument.Open(FileUpload1.PostedFile.InputStream, false))
            {
     
            }