If you get an error like

$ webpack --progress --config resources/assets/build/webpack.config.js
Error: Failed because of a stylelint error.

at linter.then (/Users/jasper/code/alarsite/wp-content/themes/alar/node_modules/stylelint-webpack-plugin/lib/run-compilation.js:39:14)
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

And you wonder what the issue is just run the following command

yarn run lint:styles

That will tell you what the issue is.