Validation Control are not working on live server [Google Cloud] with VS 2015 (Asp.Net with C#)

On my local machine everything is working fine...but on live server validation control are not working. [I am using Google Cloud (Windows Server 2019)].

I am using VS 2015 (Asp.Net Web Form with c#))

my code is here...

 <asp:TextBox ID="txtFirstName" runat="server" placeholder="Your First Name" MaxLength="25"  ></asp:TextBox>
 <asp:RequiredFieldValidator ID="rfvFirstName" runat="server" ControlToValidate="txtFirstName" Display="Dynamic" ErrorMessage="First Name is required." SetFocusOnError="True" ValidationGroup="v" ForeColor="Red"></asp:RequiredFieldValidator>
  <asp:Button ID="btnSubmit" runat="server" Text="Submit" ValidationGroup="v"  />

I read some articles related to this. According to them I added below code in my web.config file but still issue are same...

 <add key="ValidationSettings:UnobtrusiveValidationMode" value="None" />  <!--Using 4.5 Version only-->
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