Why does my static mesh have so many problems after I create it from a box brush?

So I am using a box brush to create my floor.

I need to be able to set custom collision channels on it, so I will now make it into a static mesh.

Now the floor is extremely dark, and I don’t know why. It says “Object has wrapping UVs.”

I also can’t get a nav mesh to work on the static mesh. It just never generates the navigation field.

(The nav mesh is currently selected)

Is there a different way to go about this? Before I was just using the default Cube and putting textures on it, but with the box brush I can more precisely change the length, width, and height of the shape; and the texture isn’t stretched out on the brush like it is on the default cube.

your question is also rather the answer.
You have so many problems because you used a box brush to create your mesh.

the bsp system in ue4 is NOT made to be an alternative to modelling in a 3d program like blender, modo, 3dsmax, maya, etc.
Its uv’s will be hideous, its normal alignment atrocious, and poor control over uv-unwrapping.

But even when I create a static mesh out of my brush object, the nav mesh never generates the navigation field. (the green part). Is that not a bug? Or am I just doing something wrong? What’s the difference between a static mesh created from a box brush and a static mesh imported from 3ds max?