RequireFieldValidator nor working in Client-Side in my project

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C#

validation

client-side

Hello guys, I really need your help. All Validation controls on every page in my project don't work in client-side. the button_Click event fires immediately the button is clicked even when the validation isn't true. I put the code below in my project and in a new project, it works perfectly in client-side in the new project. Is something wrong with my App.Settings or Web.config? Please help.
Here's the code

<asp:TextBox ID="TextBox1" runat="server">
<asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ControlToValidate="TextBox1" ErrorMessage="RequiredFieldValidator" ValidationGroup="A">


<asp:TextBox ID="TextBox2" runat="server">


<asp:Button ID="Button1" runat="server" OnClick="Button1_Click" Text="Button" ValidationGroup="A" />

My code behind is...

protected void Button1_Click(object sender, EventArgs e)
        {
            TextBox2 = TextBox1;
        }

I'd appreciate a quick and favorable response. Thanks.