Just for asking…
This challenge was free at the beginning of the year, I had started to solve it but I left it for later.
—> Today I saw it but as I said it’s not free
Best regards.
All Challenges were actually supposed to be moved behind the paywall over the Summer when we reconfigured our Free and Paid offerings, but we missed these legacy Challenges. The team has been moving them over gradually since then. I had asked them at the time to slow-walk the process because you seemed to be actively engaging with them, but they will all be paywalled or retired eventually.
Might I recommend purchasing a subscription? Always a great choice 
Thank you very much for the information.
Best regards