Why is entity framework throwing this error?

ex.Message = "Unable to update the EntitySet 'ExternalEmployees' because it has a DefiningQuery and no <UpdateFunction> element exists in the <ModificationFunctionMapping> element to support the current operation."

if(externalEmployeeDataGrid.IsVisible)
                {
                    ExternalEmployee selectedEmployee = (ExternalEmployee)externalEmployeeDataGrid.SelectedItem;
                    ExternalEmployee ExtEmpContext = this.context.ExternalEmployees.Find(selectedEmployee.ID);

                    if(ExtEmpContext != null)
                    {
                        ExtEmpContext.BadgeNo = selectedEmployee.BadgeNo;
                        ExtEmpContext.EmployeeName = selectedEmployee.EmployeeName;
                        ExtEmpContext.EmployeeType = selectedEmployee.EmployeeType;
                        // ExtEmpContext.Status = selectedEmployee.Status;


                        
                        context.SaveChanges();
                    }
                }

public ExternalEmployee newExternalEmployee { get; set; }

       NewMealPlanEntities context = new NewMealPlanEntities();
       CollectionViewSource externalEmployeesViewSource;

       public ExternalEmployeesWindow()
       {
           InitializeComponent();

           externalEmployeesViewSource = ((CollectionViewSource)FindResource("externalEmployeeViewSource"));

           DataContext = this;
       }

       private void Window_Loaded(object sender, RoutedEventArgs e)
       {

           //System.Windows.Data.CollectionViewSource externalEmployeeViewSource = ((System.Windows.Data.CollectionViewSource)(this.FindResource("externalEmployeeViewSource")));
           // Load data by setting the CollectionViewSource.Source property:
           // externalEmployeeViewSource.Source = [generic data source]

           context.ExternalEmployees.Load();
           externalEmployeesViewSource.Source = context.ExternalEmployees.Local;