Error: the process cannot access the file because it is being used by another process.

I have created a file watcher processor which transforms xls file to csv file once the file gets dropped in the folder.

While doing so i get the error The process cannot access the file '\\somepath\someotherpath\user\FileName_2018.xls' because it is being used by another process.

private static string sourcePath = ConfigurationManager.AppSettings["Excel"];
       private static string destination = ConfigurationManager.AppSettings["CSV"];
       private static string xlsFileName = string.Empty;
       static void Main(string[] args)
       {
           FileSystemWatcher watcher = new FileSystemWatcher
           {
               Filter = "*.xls",
               Path = sourcePath,
               EnableRaisingEvents = true
           };
           watcher.Created += new FileSystemEventHandler(Watcher_Created);
           Console.WriteLine($"FileSystemWatcher ready and listening to changes in :\n\n{sourcePath}");
           Console.Read();
       }
       static void Watcher_Created(object sender, FileSystemEventArgs e)
       {
           xlsFileName = e.Name;

           var filePath = sourcePath + xlsFileName;
           var extension = Path.GetExtension(filePath);

           if (extension.ToLower() == ".xls")
           {
               Console.WriteLine($"\n{xlsFileName} file is saved and ready to be processed.");
               PerformETL(xlsFileName);
           }
       }

       private static void PerformETL(string xlsFile)
       {
           try
           {
               var isCSVFileCreated = false;
               var filePath = sourcePath + xlsFile;
               var extension = Path.GetExtension(filePath);
               var fileNameWithoutExtension = Path.GetFileNameWithoutExtension(xlsFile);

               if (extension.ToLower() == ".xls")
               {
                   IExcelDataReader excelReader;
                   DataSet dataset;

                   using (FileStream stream = File.Open(filePath, FileMode.Open, FileAccess.Read))
                   {
                       excelReader = ExcelReaderFactory.CreateBinaryReader(stream);
                       dataset = excelReader.AsDataSet();
                   }

                   DataTable dtTemplate = dataset.Tables[0];

                   using (var output = File.CreateText($"{destination}{fileNameWithoutExtension}.csv"))
                   {
                       for (int i = 5; i < dtTemplate.Rows.Count; i++)
                       {
                           // 7 spaces
                           output.WriteLine($"{dtTemplate.Rows[i][3].ToString().Trim()},{dtTemplate.Rows[i][4].ToString().Trim().PadLeft(9, '0')}");
                       }
                   }

                   isCSVFileCreated = true;
               }
               else
               {
                   isCSVFileCreated = false;
               }

               if (isCSVFileCreated)
               {
                   Console.WriteLine("File is extracted and transformed to space delimited file format");
               }
               else
               {
                   Console.WriteLine("File is extracted and transformed to space delimited file format");
               }

               DeleteExcelFile(filePath);
           }
           catch (Exception ex)
           {
               Console.WriteLine($"error: {ex.Message}");
           }
       }

       private static void DeleteExcelFile(string filePath)
       {
           var fileName = Path.GetFileName(filePath);
           if (File.Exists(filePath))
           {
               File.Delete(filePath);
           }
           Console.WriteLine($"{fileName} is deleted.");
       }