Make sure that you are done the following things
For enabling client side validation, we required to include the jQuery min, validate & unobtrusive scripts in our view or layout page in the following order
<script src="@Url.Content("~/Scripts/jquery-1.6.1.min.js")" type="text/javascript"></script>
<script src="@Url.Content("~/Scripts/jquery.validate.js")" type="text/javascript"></script>
<script src="@Url.Content("~/Scripts/jquery.validate.unobtrusive.js")" type="text/javascript"></script>
We can enable and disable the client-side validation by setting the values of ClientValidationEnabled & UnobtrusiveJavaScriptEnabled keys true or false. This setting will be applied to application level.
<appSettings>
<add key="ClientValidationEnabled" value="true"/>
<add key="UnobtrusiveJavaScriptEnabled" value="true"/>
</appSettings>