PostCSS "Loading PostCSS Plugin failed" in CI

PostCSS resolves each plugin named in its config and instantiates it. "Loading PostCSS Plugin failed" means one plugin could not be required or threw during setup, naming the plugin and a nested cause.

What this error means

The build fails with "Loading PostCSS Plugin failed: Cannot find module 'postcss-nested'" or a thrown error from inside the plugin.

postcss

Loading PostCSS Plugin failed: Cannot find module 'postcss-nested'
(@/home/runner/work/app/app/postcss.config.js)

Common causes

The plugin is not installed in CI

A plugin referenced in postcss.config.js is not in package.json, so a clean install never fetches it.

The plugin name is misspelled or wrong format

A typo in the plugin name, or passing a plugin where PostCSS expects a factory call, makes loading fail.

How to fix it

Install the named plugin

Read which plugin the error names.
Add it to devDependencies and reinstall.
Re-run the CSS build.

Terminal

npm install --save-dev postcss-nested

Fix the plugin reference in config

Reference each plugin by its exact package name and call factory plugins as functions where required.

postcss.config.js

// postcss.config.js
export default {
  plugins: { 'postcss-nested': {}, autoprefixer: {} },
};

How to prevent it

List every PostCSS plugin in devDependencies.
Use exact package names in the PostCSS config.
Run the CSS build in CI on a clean install to catch missing plugins.