I hope this solves your problem.

1. Check if the .env.local file is correctly located in the root.
2. Use console.log(process.env) to debug which variables are available.
3. Ensure dotenv is installed and correctly loaded if running outside Next.js.
4. Ensure you are not adding the NEXT_PUBLIC_ prefix to MONGODB_URI, as it is meant for server-side usage.

Thanks. Good Luck!