I changed Parentid to Whatid and it worked fine, and i changed title to subject.  But it still is only giving me 69%, which was what my other test class was giving me. I have highlighted in Red the parts that I cant figure out how to test.

Trigger UniqueTask on Task(before insert, before update){
    
    //The map... key:Id, value: list of tasks
    Map<Id, List<Task>> whatIdsMap = new Map<Id, List<Task>>();
    //For each task being inserted, updated
    for(Task t : trigger.new){
        //If this task has a WhoId...
        if(t.whatId != null){
           
           
            if(!whatIdsMap.containsKey(t.WhatId)){//If not found
                //Create temporary list for tasks
                List<Task> temp = new List<Task>();
                //Add current task to temp list
                temp.add(t);
                
                whatIdsMap.put(t.WhatId, temp);
            }else{//If found
                
                whatIdsMap.get(t.WhatId).add(t);
            }//End if else
        }//End if
    }//End for
    
   
    for(Account Acc : [Select Id,Account_Owner_Role__c, Days_with_no_Activity__c from Account where Id in :whatIdsMap.keySet()]){
        //Get the tasks for this account and iterate through them
        //Update field value
        if(acc.Days_with_no_Activity__c >120){
        for(Task t :whatIdsMap.get(acc.Id)){
            If(t.Task_Owner_Role__c==acc.Account_Owner_Role__c&&t.Status=='Completed'){
            t.Unique_Activity__c = True;
        }//End for
    }
    }
    }//End for
}//End trigger

Write it like this

@isTest

Class TestUniqueTaskOnTriggerTest {

@istest static void test(){

account a = new account(Name = 'test',Days_with_no_Activity__c =121);

insert a;

task t = new task(whatId=a.id, title = 'Test Task');

test.starttest();

insert t;

test.stoptest();

}

}

If this post is helpful please throw Kudos.If this post solves your problem kindly mark it as solution.

Thanks

I tried that with one of my other classes, but i get an error.

Error: Compile Error: Field is not writeable: Account.Days_with_no_Activity__c at line 4 column 65