Network Access: Make sure your current IP address is whitelisted in MongoDB Atlas. You can set it to allow access from anywhere (0.0.0.0/0) for testing.
Credentials: Verify that the username and password in your connection string are correct.
Connection String Format: Ensure your connection string follows the format: mongodb+srv://username:password@cluster.mongodb.net/database
Environment Variables: Check that the MONGODB_URI environment variable is correctly set in your Vercel project.
Database User: Confirm that the database user has the appropriate permissions (at least readWrite).