Hello again
I took a look at your problem:
- {l Code}: {l Select All Code}
[warn ] Irrlicht: Could not open file of texture: checker.png
As I expected, STK doesn't find your textures. You have to copy the texture file (checker.png) into your kart directory. I first thought this would solve the problem... but:

You notice the graphical errors in the background. There's still something wrong. (Another funny image
here)
So I checked your mesh:
- Faces seems good and well done
- No doubles (which is good)
- Proper calculated normals
- UV ok too
Two things to correct:
- Apply Object Transform: The transformation values should always be reset to standard values befor exporting. Otherwise STK could misinterpret the scale, rotation or location of your object.
- Only use one UV map:Unfortunately, STK cannot handle more than one applied UV map. For texturing you're forced to use only one UV map for every object. This is what caused this graphical glitch / vanishing of your model.
Note: In the image, I also deleted your Vertex color layer since you haven't used it anyway. But it is not mandatory of course.

Apply Object Transform: Check all of the options
Location, Rotation and Scale
Only 'UVMap' left over.

Don't forget to apply the object transform to your armature too (for future animating). Or you will end up with something like this:


That's it! Nice work by the way, new contributors are always welcome!
The edited Blender file is attached (if you really need it).
--> I raised your car a little, because STK sometimes has trouble with too much lowered cars.