Hi,

You can bind the dropdownlist with the above code. Maybe you have bound the dropdownlist or other control in the .aspx page. And your error can be caused by it. You can remove it and bind the dropdownlist from the code behind. You can also bind the dropdownlist in the .aspx page. You can refer to the below code.

<asp:CreateUserWizard ID="CreateUserWizard1" runat="server">
            <WizardSteps>
                <asp:CreateUserWizardStep ID="CreateUserWizardStep1" runat="server">
                    <ContentTemplate>
                        <asp:DropDownList ID="DropDownList1" runat="server">
                        </asp:DropDownList>
                         
                         <asp:DropDownList ID="DropDownList2"  runat="server" DataTextField="ProductID"
                             DataValueField="ProductID" DataSourceID="SqlDataSource1"  />
                            <asp:label runat="server" associatedcontrolid="UserName" id="UserNameLabel">
                             User Name:</asp:label>
                             <asp:TextBox ID="UserName" runat="server"></asp:TextBox>
                             <asp:textbox runat="server" textmode="Password" id="Password"></asp:textbox>
                             <asp:textbox runat="server" textmode="Password" id="ConfirmPassword"></asp:textbox>
                             <asp:textbox runat="server" id="Question"></asp:textbox>
                              <asp:textbox runat="server" id="Answer"></asp:textbox>
                              <asp:textbox runat="server" id="Email"></asp:textbox>
                        <asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:NORTHWNDConnectionString %>"
                SelectCommand="SELECT [ProductID] FROM [Products]" ></asp:SqlDataSource>
                    </ContentTemplate>
                </asp:CreateUserWizardStep>
                <asp:CompleteWizardStep ID="CompleteWizardStep1" runat="server">
                </asp:CompleteWizardStep>
            </WizardSteps>
        </asp:CreateUserWizard>

The below code is in the code-behind.

        protected void Page_Load(object sender, EventArgs e)
        {
            string[] array = new string[] { "apple", "orange", "banana" };
            DropDownList ddl = CreateUserWizard1.CreateUserStep.ContentTemplateContainer.FindControl("DropDownList1") as DropDownList;
            ddl.DataSource = array;
            ddl.DataBind();
        }