Bring the box in a container with the enemy.
You only need 1 box in the layout, the others are auto created.
Erase that last pick based on IID.
Now your code will work (i think).
IID is the index of an instance of an object in the Selected Objects List (picklist)
That 'On collision' picks 1 Enemy, its IID is zero (its index in that picklist)
There is no Box prepicked. So they are ALL in the picklist. Box with IID = zero is always the first one off all objects in that picklist.