display validation error for dropdownlist linked with DB

Question

This the code I tried but the validation message not appeared  

 

<asp:DropDownList ID="DropDownList2" runat="server" EnableViewState="true" DataSourceID="SqlDataSource6" DataTextField="MName" DataValueField="MId" Width="130px" Height="25px"><asp:ListItem>Select ModuleID</asp:ListItem>

</asp:DropDownList>

<span runat="server" style="color:red; font-size:20px">*</span>

<asp:RequiredFieldValidator ErrorMessage="Module ID is a required field." id="RequiredFieldValidator3" ForeColor="Red" ControlToValidate="DropDownList2"

runat="server" ValidationGroup="click" />

<asp:SqlDataSource ID="SqlDataSource6" runat="server" ConnectionString="<%$ ConnectionStrings:ConnectionString %>"

SelectCommand="SELECT [MId], [MName] FROM [Modules]"></asp:SqlDataSource>