Hi SaChie,
Please take reference the below code and correct your code.
HTML
<div>
Name:
<asp:TextBox ID="txtName" runat="server"></asp:TextBox>
<asp:RequiredFieldValidator ID="rfvtxtName" runat="server" ControlToValidate="txtName"
ErrorMessage="Please Enter Name"></asp:RequiredFieldValidator>
<br />
<asp:Button ID="btnValidate" runat="server" Text="Submit" OnClientClick="Validate();" />
</div>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<script type="text/javascript">
function Validate() {
var name = document.getElementById("txtName").value;
if (name == "") {
$("#txtName").css("border", "1px solid red");
$("#txtName").focus();
}
}
</script>
Screenshot
