Project: https://todo-list-daniel-31d4167bc658.herokuapp.com/
This project was running normally on my Local machine but I suffered a lot with this error after Heroku Deployment. After a deep research, I found the basic solution.
To solve this problem you can't forget to insert mongoose.connect() method inside an async function.
I made this complete ToDo list using MVC architecture, Node/Express.js, mongoose, ejs, bootstrap, messages, modal, and tooltip.