face is flickering in blender and invisible in unity and i cant add a texture I have an invisible face in unity, and a black/flickering face in blender, I've looked it up,

but cant seem to find answer, so I have no idea how to fix it.  Here is a link to my blend file

I have tried things like normalizing, it did fix parts of my problem, but this is the only one left and I can't seem to add a texture to the cube for some reason, as it just appears white.
You have some overlapping faces. They will never render correctly. And in unity it's even worse because it uses only single-sided faces. Follow the link given by  Duarte Farrajota Ramos, there are some tips how to get rid of them.
Also try to revise your mesh to avoid face overlapping. In your case it's simpler to model it once again. Just use loopcuts (Ctrl+R) to create points, where the new faces will begin, instead of overlapping them.

look this video.