Using a fresh Hugo install, and following the docs for PostCSS, the following results in a permalink of http://localhost:1313/
(no errors).
{{ $css := resources.Get "css/main.css" }}
{{ $style := $css | resources.PostCSS }}
<link rel="stylesheet" href="{{ $style.Permalink }}">
$css.Permalink
works as expected (http://localhost:1313/css/main.css
)
postcss-cli
is installed globally, as per docs.