Pop-up message error

Question

Hi currently I am doing a online calendar project.
There ia a drop-down list tied to reminder(label) and when i select any of the reminder options in the ddl and
click on downlaod pdf button, there should be a message box that tells users they are not allow to select any of the reminder options
for PDF.
This is the code that i have entered:

 

protected void downloadPDFBtn_Click(object sender, ImageClickEventArgs 
e)
    {
        //calls 
the method to create the datatable
        
GetCustomMadeDataTable();
        //calls 
the method to generate the pdf from the 
datatable
        GeneratePDF(table);

        //popup error message when staff 
choose one of the field under the Reminder field and click on the "download PDF" 
button.
        if 
(DropDownList1.SelectedItem.Value != 
null)
        
{            

            
MessageBox.Show("There is no timetable available 
for");
            
Response.Redirect("DownloadTimetable.aspx");                    

        }

Thanks ur help is deeply appreciated. Hope to hear from u soon!