@YannickPezeu alright was I said above is not fully correct and I also got the same error. Here’s what I did and how I resolved it:
From a fresh project:
dfx new myproject
cd myproject
dfx start --background
dfx deploy
At this point, you can already test if you open http://canister-id.localhost:8000 in your browser
You can find the canister-id with the dfx command or in the file .dfx/local/canister_ids.json (the myproject_assets).
Then, I run npm run start, I get the error Unable to load '@webpack-cli/serve' command too!
To solve it, I installed a newer version of the webpack-cli
npm rm webpack-cli && npm i webpack-cli
Then I was able to run npm run start and open the app in the browser at the adress http://localhost:8080 and it was alright too