Install node modules on changing into both backend folder and frontend folder
cd backend
npm i
cd frontend
npm i
Insert your connection string in app.js in the backend folder to a mongodb Atlas connection string or local db connection string mongodb://127.0.0.1:27017/
Start the node server within backend folder
node app
Start the react dev server within frontend folder
npm run start
Make sure the react dev server is running on port 3000