****Connecting Mongoose to a database****

Sorry for not seeing this, but your MongoDB connection string. It should look like this:

const url = "mongodb+srv://NOT_MY_USERNAME:NOT_MY_PASSWORD@NOT_MY_DB.abcde.mongodb.net/DB?retryWrites=true&w=majority";

const client = new MongoClient(url, {
    useNewUrlParser: true,
    useUnifiedTopology: true,
});

You can see the instructions for setting up a MongoDB Atlas username and password here.

I hope that helps.