When Opportunity closed task should not be create?

Question

How to stop the creating a task when opportunity stage = closed won? If we trying to create a task for closed opportunities we should fire a Error message like "You can not add a task on opportunity if the status is Completed".

we have written a trigger but not working properly.Please find the below trigger and let us where we made the mistakes.

trigger CreateTask on Opportunity(after insert,after update,before insert){

    List<Opportunity> listOpp = Trigger.new;
    List<Task> listTask = new List<Task>();


    if(Trigger.isInsert){
        for(Opportunity opp:listOpp){               
            if(opp.Need__c != null && opp.StageName=='AC5-Closed Won'){
                Task t = new Task();
                t.OwnerId = opp.OwnerId;
                t.WhatId = opp.Id;
                t.Subject ='Help' + ': ' +opp.Need__c + ': '  + opp.Name; 
                //t.ActivityDate = opp.Need__c;
                t.ActivityDate = System.Today()+7;  
                t.Description = opp.Description__c;
                t.Status = 'In Progress';
                t.Priority = 'Normal';
                listTask.add(t);
            }
            else{ 
                t.addError('You can not add a task on opportinty if the status is Completed or Closed Lost or Abandon');
            }

        }
    }
}

Answer 1

From the description it appears you may have two requirements

1 - Automatically create Tasks on Open Opportunities (but not closed)

trigger CreateTask on Opportunity(after insert,after update){
 // Adds Tasks to each open Oppo on insert or relevant update
 List<Task> tasksToUpdate = new List<Task>();

 for(Opportunity opp: Trigger.new){
   if ( opp.StageName != 'AC5-Closed Won' &&  // consider only open Oppos
        (Trigger.isInsert ||                  // newly inserted or
        (Trigger.isUpdate &&                  // on update, some relevant fld has changed
           opp.Foo__c != Trigger.oldMap.get(opp.id).Foo__c ||
           opp.Bar__c != Trigger.oldMap.get(opp.id).Bar__c
           ..)  // wasn't clear what role Need__c played but could be added to this if stmt
        ) 
      )               
        tasksToUpdate.add(new Task(
            OwnerId = opp.OwnerId,
            WhatId = opp.Id,
            Subject ='QMIHelp' + ': ' +opp.Need__c + ': '  + opp.Name, 
            ActivityDate = System.Today().addDays(7),  
            Description = opp.Description__c,
            Status = 'In Progress',
            Priority = 'Normal');
 }
 insert tasksToUpdate;  // no need to check for empty list       
}

2 - Prevent Tasks from being added to Closed-won Opportunities

Exercise left for the reader - but basically you will need a before insert trigger on Task. There's a code sample here on Developer Forum that is helpful