Thank you for your detailed responses despite the poor description from my side, though I’m not sure if I can make any change with them 
I hope I’ll be able to really explain the issue this time
(Now) I found out that the moment I set “BP_Fire” Variable a type “Blueprint Effect Fire” —> Object reference, its Default Value can’t be changed after. I don’t know if this is just how UE works or if it’s a bug. I supposed to be able to select there an instance of Blueprint_Fire_Effect from starter pack, so like when the barrel gets a shot, it fires, but again, the drop-down menu to set this is inactive.
Different Internet sources say that it should actually work, or maybe I should change the Var. type to Class Reference, but in this case the Barrel’s Event Graph can’t be executed in the way it is (because it uses the specific nod Get: BP_Fire Variable available for Object Reference only). I can’t really use the Runtime because it’s a bit too advanced for my noob level and I believe this exercise must work the way it is?
I already placed the actors in the level, and I checked Editable/Editable when Inherited. I tried setting all the insides before and after placing the actor on the level. And I reread and redid the whole thing several times even on different versions.
I just want to know why it doesn’t work
The Play gives me the same error message, which is obvious yeah
