New in WCF

Getting this error at the time of compiling WCF service:

"Failed to add a service. Service metadata may not be accessible. Make sure your service is running and exposing metadata."

The service encountered an error:

An ExceptionDetail, likely created by IncludeExceptionDetailInFaults=true, whose value is:System.InvalidOperationException: An exception was thrown in a call to a WSDL export extension: System.ServiceModel.Description.DataContractSerializerOperationBehavior contract: http://tempuri.org/:ITIN_EXT_ServiceMobile ----> ..... bla, bla....

My service is:

namespace MyService
{
    using TIN.DataMobile;       

    [ServiceContract]
    public interface ITIN_EXT_ServiceMobile
    {
        [OperationContract]
        string findContact(int eid);

        [OperationContract]
        String addUser(string uID);

        [OperationContract]
        Mob_AddSessionOut Mob_AddSession(Mob_AddSessionIn pInput);
    }
}

The issue is in the last service because all is fine with the first two service.

Where:

public class Mob_AddSessionOut
    {
        public String Auth_token { get; set; }
        public Boolean Transaction_moviment { get; set; }
        public TIN.DataAccessLayer.Gen_StatusProcess StatusProcess { get; set; }    

    }

What am I doing wrong?

Thanks