Short answer: no, no one is forcing anyone to make anything paid.
HI there! You are expected to put it up for sale, but since it's a personal challenge, you can decide not to. The best way if you're hesitant is, in my opinion, to put it up for sale at a "pay what you want/can" price on itch. That way you can still define the price you think it should cost, but let people get it at any price they want. Hive Time do it that way for instance.
Hope it answered you question and hope you'll have a great time during this challenge!