resource: Add instruction to install PostCSS when missing

Fixes #5111

diff --git a/resource/transform.go b/resource/transform.go
index b2c63057a9f61e6457b04eab9fa914ecb7c01908..c8174875699aaf9c56b91e4bf31ffc49369fc729 100644 (file)
--- a/resource/transform.go
+++ b/resource/transform.go
@@ -391,7 +391,11 @@ func (r *transformedResource) transform(setContent bool) (err error) {
                                // If a prepared bundle for this transformation chain is available, use that.
                                f := r.tryTransformedFileCache(key)
                                if f == nil {
-                                       return fmt.Errorf("%s: failed to transform %q (%s): %s", strings.ToUpper(tr.transformation.Key().name), tctx.InPath, tctx.InMediaType.Type(), err)
+                                       errMsg := err.Error()
+                                       if tr.transformation.Key().name == "postcss" {
+                                               errMsg = "PostCSS not found; install with \"npm install postcss-cli\". See https://gohugo.io/hugo-pipes/postcss/"
+                                       }
+                                       return fmt.Errorf("%s: failed to transform %q (%s): %s", strings.ToUpper(tr.transformation.Key().name), tctx.InPath, tctx.InMediaType.Type(), errMsg)
                                }
                                transformedContentr = f
                                defer f.Close()