Instead of running npm install in directory and then in sub directory (client) simply, go inside project dorectory and run
npm install
This will first install dependencies in root directory and then in client directory because of post-install script in package.json which looks like this:
"scripts": {
"start": "node server.js",
"dev": "concurrently \"npm run start\" \"cd client && npm run start\"",
"postinstall": "cd client && npm install"
},
Inside project directory, simply run
npm run dev