Some nodes (such as GetGameInstance) need a valid WorldContext in order to do their task. And so if you try to call this from an object that does not provide this WorldContext (needs to override GetWorld in C++) blueprints prevent you from calling these functions.
The two solutions from here Is there a way to use Blueprint Function Libraries or an equivelent feature for non-actor blueprints? - #4 by UnrealEverything should work for you as well, unfortunately they both require some C++ code.