Error: yarn start - error Command "start" not found

Just run

yarn add react-scripts

Solved it thanks to the insight of the user: Tsvetan Ganev.

I was trying to run a command that it is not in my scripts. Specifically, yarn start is not in the scripts part of the file package.json. To solve the issue I added the following line in scripts

"start": "webpack-dev-server --mode development",

There is no start command inside the scripts of the package.json file.

"scripts": {
  "start": "some command to be run", // you need to add this line
  "test": "(eslint lib || true) && jest"
}

Maybe you want to run the test command instead - npm test / yarn test?