It looks like you’ve stumbled upon a long held rule of Unreal Engine. You cannot reference the Level Blueprint! (well. Not without causing major issues, don’t force it!)
The Level Blueprint references DOWN, so things within the level can be referenced by the LBP but not vice versa. You can, however, make an empty actor that accesses the GameMode which can then tell the Level Blueprint to do a thing!
Check out this post, it’s (mostly) still up-to-date!