There is a "Subscribe" button which takes you to https://app.codecrafters.io/pay but I wasn't savvy enough to notice it or realize what it was.

Only after starting a course did I begin to suspect that I needed to subscribe since there were a bunch of locks all over. However, it was not clear that the locks actually did anything since I could still click on those links. I guess I would have found out after completing the first step and not being able to progress. This appears to be by design, which strikes me as slightly dishonest.