What this red cross icon(error icon?) mean on a light?

Development Rendering
3

@Piroman,
Thanks for your detail explaining.

btw, do you know how to setup this type of lighting?:
59cd61c3029a0958020d572f51d739e813712933.jpeg

Seems, UE4 doesn’t use emissive material to lighting the scene. So I have to use spot light instead.