"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.