CODE EXAMPLE FOR JAVASCRIPT

Node Sass version 7.0.0 is incompatible with ^4.0.0

npm uninstall node-sass
npm install node-sass
change the "react-scripts": "4.0.0" into "react-scripts": "4.0.3" in package.json and save
npm install
npm start

or, using yarn -

yarn remove node-sass
yarn add --dev node-sass
as above
yarn install
yarn start