Re: The field Assignment member put to required filed in service call
You can't make "assignment;member" required field for "Service Call Status" through Security/Prevention/Required fields/Service Call.
However you can create a UI rule that will be evaluated before the item is saved and Status is Completed or Closed and Assignment;member is empty. As action you will select "User Notification Message" and make sure with Severity "Error" in order to make mandatory to user to fill this field. If Message Notification Severity is Information or Warning the user will be able to ignore the requirement.