Exercise Tracker Cannot Connect MongoDB

I need some help connecting MongoDB to my project. I am trying to connect using MongoDB's native driver to mongoose for free code camps Excercise Tracker project, and I continue to get the console error:

(node:7251) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): TypeError: Parameter "url" must be a string, not function


I was able to connect my other applications to Mongoose using the cluster key, but I have been tackling this project for days now with no luck, and would appreciate any help in this matter. My key is in the env file, but I commented out an example key in the server.js file.

Any help on this issue would be greatly appreciated. The source code is below: