I believe this is just from changes to casting in blueprints in newer versions of the engine, you don’t have to cast now in this case, if you remove it and just use the Array Element on the loop it should work
This image shows what I’ve done to the network based on your description. I do not get warning errors, however it does not work as intended; There is no effect, the 3 Blueprint_Effect_Fire objects in the level do not Deactivate.
The first time they use Get All Actors of Class in tutorial, they use Blueprint_Effect_Fire as the class, but not in the lower example.
Finding just the emitter class would disable emmiters in the world, but not ones inside blueprints as they won’t be found. So in this case you should find blueprint effect fire. But look again at the tutorial, I thinks it’s two different examples there and you have mixed them up