Hi !
I am currently working on this project as well. I have been searching through the forums to find answers. The course does a poor job of preparing us for this project. People have complained about it for quite a while now but the course has not been updated. Most people who have completed this project have either found other tutorials or have ‘reverse engineered’ the example code.
I found this prior post which may be helpful as it distills the portion of the example code that actually draws the map:
This project should either be removed from the course or the course should be updated to include this portion of D3.