# Fix a PostCSS Webpack error ruleSet rules oneOf... etc etc

> Learn how to fix the Next.js PostCSS webpack ruleSet oneOf error and Cannot find module tailwindcss, caused by a stray postcss.config.js in a parent folder.

Author: [Flavio Copes](https://flaviocopes.com/about/) | Published: 2022-05-29 | Topics: [Tools](https://flaviocopes.com/tags/tools/) | Canonical: https://flaviocopes.com/fix-postcss-webpack-ruleset-oneof/

Some students of my [Bootcamp](https://bootcamp.dev) had a problem that I couldn't replicate in any way.

Upon running `npm run dev` in a [Next.js](https://flaviocopes.com/nextjs/) project, they had this error:

```
➜  rest-api git:(main) npm run dev

> rest-api@0.1.0 dev
> next dev

ready - started server on 0.0.0.0:3000, url: http://localhost:3000
info  - Loaded env from /Users/flaviocopes/dev/bootcamp/rest-api/.env
wait  - compiling...
error - ./node_modules/next/dist/build/webpack/loaders/css-loader/src/index.js??ruleSet[1].rules[3].oneOf[8].use[1]!./node_modules/next/dist/build/webpack/loaders/postcss-loader/src/index.js??ruleSet[1].rules[3].oneOf[8].use[2]!./styles/globals.css
Error: Cannot find module 'tailwindcss'
```

In the last line `tailwindcss` appears but also `autoprefixer` happened.

Or, this error too: `Error: Your custom PostCSS configuration must export a plugins key.`

Not a very helpful error!

After much debugging, the cause of the problem was discovered.

The project where the error appeared didn't have Tailwind CSS installed, but during the weeks before, we used Tailwind.

What happened was this.

They had a `postcss.config.js` file in the parent folder. Or in another parent folder in their path, not just the direct parent. Maybe their home folder.

Something like this:

```
module.exports = {
  plugins: {
    autoprefixer: {},
  },
}
```

Important: in the project that they're now, **there's no PostCSS configured**.

If you have PostCSS installed, there's no problem. Only if you don't have `postcss.config.js` in your current project root folder.

But the **tooling** of Next.js sees there's a `postcss.config.js` file in the parent folder, and executes it.

Raising this error:

```
error - ./node_modules/next/dist/build/webpack/loaders/css-loader/src/index.js??ruleSet[1].rules[3].oneOf[8].use[1]!./node_modules/next/dist/build/webpack/loaders/postcss-loader/src/index.js??ruleSet[1].rules[3].oneOf[8].use[2]!./styles/globals.css
```