Attribute is not recognized, but Namespace is added [closed]

My [DataContract] attribute is not being recognized, but the namespace System.Runtime.Serialization is added to my using .

Does anyone know why?

....
using System.Runtime.Serialization;
using System.Data.Linq.Mapping;

namespace SuporteTecnicoContracts
{
    [DataContract(Namespace = "")]
    [Table(Name="T_PDV")]
    class T_PDV
    {
        public int IDPdv { get; set; }
        public string CNPJ { get; set; }
        public string RazaoSocial { get; set; }
        public string NomeFantasia { get; set; }
        public string Endereco { get; set; }
        public string Numero { get; set; }
        public string Complemento { get; set; }
        public string Bairro { get; set; }
        public string Cidade { get; set; }
        public string Estado { get; set; }
        public string CEP { get; set; }
        public int DDD { get; set; }
        public string Telefone { get; set; }
        public Nullable<int> DDDCelular { get; set; }
        public string Celular { get; set; }
        public string Email { get; set; }
        public int QtdeCheckOuts { get; set; }
        public Nullable<int> IDTipoRede { get; set; }
        public string NomeRede { get; set; }
        public int IDTipoEstabelecimento { get; set; }
        public string EnderecoIPInstalacao { get; set; }
        public System.DateTime DataCadastro { get; set; }
        public Nullable<System.DateTime> DataAlteracao { get; set; }
        public string CaminhoLogo { get; set; }
        public string TokenAuthentication { get; set; }
        public string Distrito { get; set; }
        public Nullable<decimal> Latitude { get; set; }
        public Nullable<decimal> Longitude { get; set; }
        public bool IS_Ativo { get; set; }
        public Nullable<int> ID_Rede { get; set; }
    }

....