MongoDB is a NoSQL database. It's a non-relational database of keys-values objects.
NodeBB doesn't use SQL, so you will not use your SQL database to make NodeBB work.
In NodeBB, subcategories are referred to as child categories. In order to set up your subforum, you would create your necessary categories (one for mobile phones, and then others for android, ios, etc) and then set the parent category of your intended subforums to be your mobile phones category.
Looking forward to when we can have a "Make Forum Private" option that sets all permissions on all forums automatically so that you don't have to go about changing each one.
NPM common issue , for solving this use add swap 
