What is “plewb”? Is this the wall? If you overlap with your Character, the cast will always fail, because you are trying to cast your Character to the Wall class, which is not possible.
I recommend you to learn about classes and casting. Only because BP don’t look like code, you will still need to understand how C++ works. Otherwise you won’t get happy with BP.
For your problem:
Get a reference of the Wall that has the “Begin” event in your LevelBP by selecting it in the Level and rightclicking in the LevelBP. You should see something like “Add Reference of …” there.
Once you got that, remove the cast and the Event call. Cast the “Other Actor” to your CharacterBP Class (only to make sure that only the Character overlaps. You won’t use the casted object!).
Now, behind the successful cast, drag out a wire from the Wall Reference and call your “Begin”. Pack this begin call behind the Successful cast.