"Unable to load manifest file"

Here is the contents of this file on my linux machine (which shouldn't matter). I've removed the comments included with it for brevity.

surfaceproperties_manifest
{
    "file"      "scripts/surfaceproperties.txt"
    "file"      "scripts/surfaceproperties_portal.txt"
    "file"      "scripts/surfaceproperties_portal2.txt"
}

The file is located in my

.../steam/steamapps/common/Portal 2/portal2/scripts/

directory. I'll assume from your question that this file isn't already present there.