This error usually indicates one of two issues:

Your MongoDB Atlas IP whitelist is not set up correctly
Your environment variables (your port or MongoDB URI) are misconfigured
Here’s a checklist of common steps to resolve the problem:

1. Whitelist your IP Address in MongoDB Atlas
   Log in to MongoDB Atlas and open your Cluster.
   Go to the Network Access tab.
   Under the IP Access List, make sure to add your current IP address (or 0.0.0.0/0 if you are just testing and want to allow access from anywhere, though that’s less secure).
   Once added, wait a moment for Atlas to apply the new IP access settings.
   Re-run your application.
   Note: Ensure that the connection string in your .env or configuration file matches the connection string given in the “Connect” dialog in Atlas, including the username and password.

2. Check Your Environment Variables
   Check your PORT: It looks like your console is showing:

   Server running on port undefined

This suggests the environment variable for your port might not be set. Make sure you have something like:

typescript

const PORT = process.env.PORT || 4000;
app.listen(PORT, () => {
  console.log(`Server running on port ${PORT}`);
});

Check your MongoDB URI: Often stored in an .env file as something like:

MONGODB_URI=mongodb+srv://<username>:<password>@yourcluster.mongodb.net/<database>?retryWrites=true&w=majority

Then in your code:

typescript

import mongoose from 'mongoose';

mongoose
  .connect(process.env.MONGODB_URI as string, { /* options */ })
  .then(() => {
    console.log('Connected to MongoDB');
  })
  .catch((error) => {
    console.error('Failed to connect to MongoDB:', error);
  });