The usual cause of this is that you cannot have an Assignment Due before it was Assigned. For example if today is 10/1 and I am trying to add an Assignment that is Due 9/1, the Assigned Date defaults to today 10/1 and the Due Date displays as 9/1, therefore 9/1 will not save because it was is assigned on 10/1 which is after the Due Date of 9/1. To correct this, if you change the Assigned Date to be the same as the Due Date (in this example 9/1) you can Save the Assignment correctly.