In the root directory, type mongod in your terminal. This will start a local mongo server.
Then,
Now we must install the dependencies for the backend server to run.
In terminal, type npm i to install them.
In terminal type nodemon index.js to start the backend server.
Note : Do not start the backend server without starting a local mongo server. It will throw an error! Also, I have assumed that you have mongoDB installed locally. If not please intall it before you proceed.