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

Commande npm run build

>npm run build

> build
> encore production --progress

Running webpack ...

99% done plugins FriendlyErrorsWebpackPlugin DONE  Compiled successfully in 16271ms
                                    18:30:12

5 files written to public\build
Entrypoint app 65.7 KiB = runtime.61b1725c.js 1.36 KiB 186.ee345727.js 61.6 KiB app.b75294ae.css 30 bytes app.dd4da228.js 2.69 KiB
webpack compiled successfully

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