In the challenge description, there is the line:
" Be sure to set DATABASE in your .env file to your database’s connection string (for example: DATABASE=mongodb://admin:firstname.lastname@example.org:12345/my-project ). You can set up a free database on mLab."
You will want to follow that link to mLab , sign up, and set up a database. It should direct you to a link / connection string that is similar to mongodb://admin:email@example.com:12345/my-project. This is what you will set as DATABASE in the .env file