All that is good, but PHPStorm don’t understand the syntax it and it is angry. For we solve it, we mush install and configure postCSS support plugin
Search and install
Go to preferences/plugins/Install Jetbrains plugins, let’s write postcss support and install it.
Go to preferences/Stylesheets/Dialects, you choose PostCSS in Project CSS Dialect and then you mush add your css directory
Once the changes are applied, you can work well with postcss in phpstorm.