Yes it is important to understand how the communication between the FirstPersonCharacter and the BP_Puzzle works.
In the FirstPersonCharacter blueprint I have a Variable that is of class BP_Puzzle, this will act as a reference to the puzzle so we can communicate with it. The important thing here is that I’ve set it to be Editable (notice the eye to the right in the picture).
When we have both the FirstPerosnCharacter and the BP_Puzzle dragged out in the level we can then set that variable reference. See picture below.
This will allow us to communicate between the two blueprints, i.e when we Click F, we will do a raytrace inside the FirstPersonCharacter, if we hit the puzzle we will call the event User Clicked on Cube inside the BP_Puzzle
Hopefully this cleared some of the things up 