[Solved] Export to Excel using OpenXML Error: An object reference is required for the nonstatic field, method, or property member

You cant directly access the controls in static method. You need to pass it as parameter.

Refer the updated code.

protected void btnexcel_Click(object sender, ImageClickEventArgs e)
{
    DataSet ds = new DataSet();

    using (MySqlConnection co =
        new MySqlConnection(ConfigurationManager.ConnectionStrings["co"].ConnectionString))
    {
        using (MySqlCommand cmd =
            new MySqlCommand("sPROC_XLSX", co))
        {
            MySqlDataAdapter da = new MySqlDataAdapter(cmd);
            cmd.CommandTimeout = 2147483;
            cmd.Connection.Open();
            cmd.CommandType = CommandType.StoredProcedure;
            cmd.Parameters.Clear();

            if (ddlut.SelectedIndex > 0)
            {
                cmd.Parameters.AddWithValue("@tddlut", ddlut.SelectedValue);
            }
            else
            {
                cmd.Parameters.AddWithValue("@tddlut", DBNull.Value);
            }

            da.Fill(ds);
            ExportDataSetToExcel(ds, ddlut);
        }
    }
}

public static void ExportDataSetToExcel(DataSet ds, DropDownList ddlut)
{
    string AppLocation = @"D:\Public\ExcelFiles\";
    string guid = Guid.NewGuid().ToString().ToUpper().Replace("-", "_");

    HttpContext.Current.Response.Cookies["guid"].Value = guid.ToString();
    HttpContext.Current.Response.Cookies["guid"].Expires = DateTime.Now.AddMinutes(3);

    string filepath = AppLocation +
        DateTime.Now.ToString("ddMMyyyyHHmm") + "_" + HttpContext.Current.Response.Cookies["guid"].Value + ".xlsx";

    using (XLWorkbook wb = new XLWorkbook())
    {
        for (int i = 0; i < ds.Tables.Count; i++)
        {
            wb.Worksheets.Add(ds.Tables[i], ds.Tables[i].TableName);
        }
        wb.Style.Alignment.Horizontal = XLAlignmentHorizontalValues.Center;
        wb.Style.Font.Bold = true;
        wb.SaveAs(filepath);
        wb.Dispose();
    }

    Thread.Sleep(3000);
    HttpContext.Current.Response.Clear();
    HttpContext.Current.Response.Buffer = true;
    HttpContext.Current.Response.ContentType = "application/force-download";
    HttpContext.Current.Response.AppendHeader("Content-Disposition", "attachment; filename="
                + ddlut.SelectedValue + "_" + DateTime.Now.ToString("ddMMyyyyHHmm") + "_"
                + HttpContext.Current.Response.Cookies["guid"].Value + ".xlsx");

    HttpContext.Current.Response.TransmitFile(@"D:\Public\ExcelFiles\" +
        DateTime.Now.ToString("ddMMyyyyHHmm") + "_" + HttpContext.Current.Response.Cookies["guid"].Value + ".xlsx");

    HttpCookie cookie = new HttpCookie("ExcelDownloadFlag")
    {
        Value = "Flag",
        Expires = DateTime.Now.AddDays(1)
    };

    HttpContext.Current.Response.AppendCookie(cookie);
    HttpContext.Current.Response.End();
}