It sounds like you have Blueprintable but forgot to add the BlueprintType specifier in your class declaration? I.e.
UCLASS(Blueprintable, BlueprintType)
class RC2_API AItem : public AActor
UPDATE:
It seems the issue is that you are trying to refer to an Actor instance from a Blueprint Class Default Object. This is not possible by design. A Blueprint can be instantiated in any world but an Actor exists in a specific world, so it wouldn’t actually make sense anyway. If you want to set up the default object at the class level, you can add custom logic in the Construction Script to set the field at the instance level.