Your code passes the tests for me (I needed to fork your project, and add in my MONGO_URI). This means the issue is likely one of these:
Your database is not set up correctly. Are you sure you whitelisted all IP addresses?
Your URI is slightly incorrect.
Unfortunately, I cannot find out this for you, so I recommend you go over the instructions about setting up the database again: MongoDB and Mongoose | freeCodeCamp.org