Sorry I can not guess the right answer, can I have it explained please??

Haytham, first, you need to create a condition so the loop will run once for each element in the array. Treehouse gave you a loop counter, starting at 0. So the loop needs to run until that counter equals the index of the last element in the array, which is numbers.count - 1, i.e., < numbers.count. Then, each time through the loop, you need to add the value of the element to sum: sum += numbers[counter]. To get the value of each element you need to use subscript notation: numbers[counter]. Finally, you need to increment counter each time through the loop so you traverse the array: counter++. Note that sum is what is called an accumulator because it occurs on both sides of the assignment operator: sum = sum + numbers[counter], which is the longer version of sum += counter.

while (counter < numbers.count) {
  sum += numbers[counter]
  counter++;
} 

thanks but it does not work and I am getting error saying "Bummer! Make sure your condition allows for iteration over all elements in the numbers array and check your calculation of sum"

Not sure what to say. I just tested it again and Treehouse came back with "Well, done!.." Suggestion. Close your browser completely, and then go back to the Challenge page and then replace everything in the code window with the following:

let numbers = [2,8,1,16,4,3,9]
var sum = 0
var counter = 0

// Enter your code below
while (counter < numbers.count) {
  sum += numbers[counter]
  counter++;
} 

I found on several occasions that if I did something wrong the browser seemed to cache my wrong answer and subsequent corrections were ignored. It has only happened a couple of times, although I've made many more errors than that, so it may be something else. But it doesn't hurt to start with a truly clean slate! Just make sure that there's no other code than the above after you copy it and paste it in.