C# - Server cannot set content type after HTTP headers have been sent

I trying to download SSRS report in excel format from my MVC application.

    Response.Clear();
    Response.Buffer = true;
    Response.Charset = "";
    Response.Cache.SetCacheability(HttpCacheability.NoCache);
    Response.ContentType = "application/vnd.ms-excel";
    Response.AppendHeader("Content-Disposition", "attachment; filename=" + reportName + "." + extension);
    Response.BinaryWrite(result);
    Response.Flush();
    Response.End();

But i am getting this error as -

System.Web.HttpException (0x80004005): Server cannot set content type after HTTP headers have been sent.

I tried changing Response.Buffer to BufferOutput but still getting the same error. What am I missing here?

In the code below, i tried to download an excel file as -

        Response.Clear();
        Response.Buffer = true;
        Response.Charset = "";
        Response.ContentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
        Response.AddHeader($"content-disposition", "attachment;filename=" + "{reportFileName}" + ".xlsx");
        using (System.IO.MemoryStream MyMemoryStream = new MemoryStream())
        {
            MyMemoryStream.Write(result, 0 , result.Length);
            MyMemoryStream.WriteTo(Response.OutputStream);
            Response.Flush();
            Response.End();

            return File(MyMemoryStream.ToArray(), "application/octet-stream");
        }