GeeksGoneBad is right, unreal doesn’t really know what the actor is and what variables it has in it, so you have to do a Cast ,witch is basically a check to see if the actor is actually a ball or not.
Here is some documentation on how to do that:
https://docs.unrealengine.com/en-us/Engine/Blueprints/UserGuide/CastNodes
One more thing, how are you setting the variables from the Spawned Balls array? Are you adding them to the array as you spawn them? Or by using get all actors of class?
If you use get all actors of class, and you select ball class, promote the array to a variable, then you can modify their distance value without needing to cast, because you are already telling unreal to search for the ball class.
Hope that makes sense, casting took a while for me to wrap my head around it.