why my validation is not working? asp.net mvc

I tried to put a ValidationMessageFor in my view to control data but it's not working and data goes to my post action.

Here is my model class:

 public enum CustomerType
    {
        JuridicalPerson,
        NaturalPerson
    }
    public class Customer
    {
        [Key]
        [DisplayName("ID")]
        public int CustomerId { get; set; }
        [Required(ErrorMessage = "Please enter your first name")]
        [DisplayName("First name")]
        [DataType(DataType.Text)]
        public string FirstName { get; set; }
        [Required(ErrorMessage ="Please enter your last name")]
        [DisplayName("Last name")]
        [DataType(DataType.Text)]
        public string LastName { get; set; }
        [Required(ErrorMessage ="Please choose your customer type")]
        [DisplayName("Customer type")]
        public CustomerType CustomerType { get; set; }

and here is my create customer view:

@model Contracts.Models.Customer

@using (Html.BeginForm())
{
    <div class="box">
        <div class="input-container">
            @Html.LabelFor(model => model.FirstName)
            <br />
            @Html.EditorFor(model => model.FirstName)
            @Html.ValidationMessageFor(model => model.FirstName)
        </div>
        <div class="input-container">
            @Html.LabelFor(model => model.LastName)
            <br />
            @Html.EditorFor(model => model.LastName)
            @Html.ValidationMessageFor(model => model.LastName)
        </div>
        <div class="input-container">
            @Html.LabelFor(model => model.CustomerType)
            <br />
            @Html.EnumDropDownListFor(model => model.CustomerType, "Select a type", new {@class ="input-container" })
            @Html.ValidationMessageFor(model => model.CustomerType)
        </div>
        <input class="btn" type="submit" value="Create" />
    </div>
}

and finally this is my customer controller and its action for create a customer:

 [HttpPost]
        public ActionResult Create(Customer customer)
        {
//db is my database context
            db.Customers.Add(customer);
            db.SaveChanges();
            return RedirectToAction("Index");
        }

actually with html helper validation in my view ,empty inputs should give an error and not goes to my action but is's not working

I'll appreciate if you help me