Hii there Trailhead Community , I am facing an issue with this module below

 https://trailhead.salesforce.com/content/learn/modules/apex_triggers/apex_triggers_bulk 

This is the code I have typed 

trigger ClosedOpportunityTrigger on Opportunity (after insert,after update) {          List<Task> taskList=new List<Task>();       for(Opportunity Opp:Trigger.New){         if(Trigger.isInsert || Trigger.isUpdate){           if (opp.StageName =='Closed Won'){             task t = new task();             t.whatID = opp.id;              t.Subject='Follow Up Test Task';             taskList.add(t);           }         }     }     

    if(taskList.size()>0)         insert taskList;  }

And I am now facing this error. I hope there is anyone to help me out with this error :)