Error Message: The INSERT statement conflicted with the FOREIGN KEY constraint

public class DashboardWorkQueue : System.Web.Services.WebService
    {
        private IErrorHandling _IErrorHandling;
        private IServiceHandler.Notification.ISubscription _ISubscription;
        private INotificationLeadTime _INotificationLeadTime;
        private IServiceHandler.Audit.IAuditNote _IAuditNote;

        public DashboardWorkQueue()
        {
            _IErrorHandling = new ErrorHandling();

            var container = new StructureMap.Container(new Service.Infrastructure.StructureMapRegistry());
           
            _ISubscription = container.GetInstance<isubscription>();
            _INotificationLeadTime = new Service.Notification.NotificationLeadTime();
            _IAuditNote = container.GetInstance<iservicehandler.audit.iauditnote>();
        }

        [WebMethod]
        public int EnableDisable(Guid userID, int recordID, Model.SubscriptionSource.SubscriptionSourceEnum subscriptionSourceEnum, bool enabled)
        {
            int subscriptionID = 0;

            try
            {
                subscriptionID = Systrends.TMS.Business.ServiceHandler.DashboardWorkQueue.EnableOrDisable(userID, recordID, subscriptionSourceEnum, enabled);
            }
            catch (Exception ex)
            {
                _IErrorHandling.LogErrorNoRedirect(ex);
                throw;
            }

            return subscriptionID;
        }

        [WebMethod]
        public int Enabled(Guid userID, int recordID, Model.SubscriptionSource.SubscriptionSourceEnum subscriptionSourceEnum)
        {
            Model.Subscription subscriptionModel = new Model.Subscription();

            int subscriptionID = 0;

            try
            {
                subscriptionModel = _ISubscription.Get(userID, recordID, subscriptionSourceEnum, Model.SubscriptionOrigination.SubscriptionOriginationEnum.UserSubscription);

                if (subscriptionModel == null)
                    return 0;

                if (subscriptionModel.DesktopLink.HasValue)
                {
                    if (subscriptionModel.DesktopLink.Value == true)
                        subscriptionID = subscriptionModel.Id;
                }
            }
            catch (Exception ex)
            {
                _IErrorHandling.LogErrorNoRedirect(ex);
                throw;
            }

            return subscriptionID;
        }

        [WebMethod]
        public void Save(int taskID, int subscriptionID, string note, Guid userID, DateTime dateDue, int filingId)
        {
            Systrends.TMS.Business.ServiceHandler.Notification notification = new Notification();

            try
            {
                if (taskID == 0)
                {
                    taskID = Systrends.TMS.Business.ServiceHandler.Task.Insert(subscriptionID, note, userID, dateDue);
                    notification.CreateTaskNotification(subscriptionID, taskID, dateDue, userID);
                    AddAuditNote(filingId, userID, "", "Insert");
                }
                else
                {
                     Systrends.TMS.Business.ServiceHandler.Task.Update(taskID, note, userID, dateDue);
                     notification.CreateTaskNotification(subscriptionID, taskID, dateDue, userID);
                     AddAuditNote(filingId, userID, "", "Update");
                                         
                }
            }
            catch (Exception ex)
            {
                _IErrorHandling.LogErrorNoRedirect(ex);
                throw;
            }
        }

        [WebMethod]
        public List<Model.Task> GetList(int subscriptionID)
        {
            try
            {
                <Model.Task> list = Task.GetList(subscriptionID);

                return list;
            }
            catch (Exception ex)
            {
                _IErrorHandling.LogErrorNoRedirect(ex);
                throw;
            }
        }

        [WebMethod]
        public Model.Task Get(int taskID)
        {
            try
            {

                Model.Task model = Task.Get(taskID);

                return model;
            }
            catch (Exception ex)
            {
                _IErrorHandling.LogErrorNoRedirect(ex);
                throw;
            }
        }


        [WebMethod]
        public void DeleteTask(int taskID,int filingId,Guid userId)
        {
            try
            {
                Model.Users user = SessionWrapper.UserModel;

                Systrends.TMS.Business.ServiceHandler.Task.Delete_ById(taskID);
                if (Guid.Empty != userId)
                {
                    AddAuditNote(filingId, userId, "", "Delete");
                }
               
            }
            catch (Exception ex)
            {
                _IErrorHandling.LogErrorNoRedirect(ex);
                throw;
            }
        }

        private void AddAuditNote(int filingId, Guid userId, string createdBy, string changeBy)
        {
            string message = null;
            if (userId == Guid.Empty)
            {
                Systrends.TMS.Data.User_View userView = RepositoryEF.User.UserRepository.Get_ByUserName(createdBy);
                userId = userView.UserId;
            }
            switch (changeBy)
            {
                case "Insert":
                    message = "Inserted Work Queue note";
                    break;
                case "Update":
                    message = "Updated Work Queue note";
                    break;
                case "Delete":
                    message = "Deleted Work Queue note";
                    break;
                default:
                    break;
            }
            if (string.IsNullOrEmpty(message))
            {
                message = changeBy;
            }
            _IAuditNote.Insert(Model.AuditNote.EntityType.Filing, filingId, userId, message);
        }

    }
}