Welcome to your first lesson in cubemaps
Very early on, before you start the fine detailing in your maps, depending on what sky texture you're using your reflective surfaces will be looking for something to reflect. If they don't know what they're supposed to reflect, they'll default to the "missing" texture, the purple checkers. A very simple and rudimentary fix is putting an env_cubemap entity anywhere in your map. Run your map, and in console, type buildcubemaps