Avec la commande : npm install postcss-loader autoprefixer –dev

>npm install postcss-loader autoprefixer --dev
npm WARN config dev Please use --include=dev instead.

added 11 packages, and audited 795 packages in 4s

80 packages are looking for funding
  run `npm fund` for details

found 0 vulnerabilities

• https://symfony.com/doc/current/frontend/encore/postcss.html
• https://yoandev.co/bootstrap-5-avec-symfony-5-et-webpack-encore/