public async Task<ActionResult> GenerateReportExcel()
    {
        ExcelGenerator excel = new ExcelGenerator();
        var filePath = await excel.ReportExcelAsync(midyearReportViewModel);
        System.Web.HttpResponse response = System.Web.HttpContext.Current.Response;
        response.ClearContent();
        response.Clear();
        response.ContentType = "text/plain";
        response.AddHeader("Content-Disposition", string.Format("attachment;filename={0}.xlsx;", PdaResource.ReportFileName)); 
        response.TransmitFile(filePath);
        response.Flush();
        response.End();
        return PartialView("_MidYearReportPartial", midyearReportViewModel);
    }

这个方法inturn调用一个excel生成器方法ReportExcelAsync，如下所示

public async Task<string> ReportExcelAsync(MidYearReportViewModel _midyearAnnualviewModel)
    {
        string fileName = "MidYearReport";
        string finalXcelPath = string.Empty;
        string currentDirectorypath = new DirectoryInfo(HttpContext.Current.Server.MapPath("~/Export")).ToString();
        finalXcelPath = string.Format("{0}\\{1}.xlsx", currentDirectorypath, fileName);
        if (System.IO.File.Exists(finalXcelPath))
        {
            System.IO.File.Delete(finalXcelPath);
        }
        var newFile = new FileInfo(finalXcelPath);
        using (ResXResourceSet resxSet = new ResXResourceSet(resxFile))
        {
            using (var package = new ExcelPackage(newFile))
            {
                ExcelWorksheet worksheet = package.Workbook.Worksheets.Add(resxSet.GetString("ReportMYMidYearExcelSheetName"));
                for (int i = 1; i <= header.Count(); i++)
                {
                    worksheet.Cells[1, i].Value = header[i - 1];
                    worksheet.Cells[1, i].Style.Font.Bold = true;
                    worksheet.Cells[1, i].Style.Fill.PatternType = ExcelFillStyle.Solid;
                    worksheet.Cells[1, i].Style.Font.Color.SetColor(Color.White);
                    worksheet.Cells[1, i].Style.Fill.BackgroundColor.SetColor(Color.DimGray);
                }
                package.Save();
            }
        }
        return finalXcelPath; 
    }

但我收到警告消息作为警告

This async method lacks ‘await’ operators and will run synchronously.
Consider using the ‘await’ operator to await non-blocking API calls,
or ‘await Task.Run(…)’ to do CPU-bound work on a background thread

。我做错了什么？我的代码工作正常，我可以下载excel。