it's probably because the chrome texture/material is somewhere near the bottom of the tree view, instead of the top.
all declared items should be placed at the top of the tree view, so that all subsequent items could use them.
pov-ray is very structured, so kpm is also. It's like a programming language: if you intend to use a variable, you have to declare it first. In your case, you want to use a variable (the chrome material) but it's not yet defined in the program structure, so the object list of kpm is empty!
hope this helps, and welcome on these boards, even if you can't read/write french, we'll try to give you helps whenever possible.