All you have to do to unlock the next free challenge is solve the previous challenge. The first 10 tutorials are designed to teach the basics of the app – how to transform data with the toolbox, how to read web page source code and run JavaScript, basic Linux commands, etc – and so they are run in order. There are 10 in total, all free, plus another one in the first territory afterwards, which teaches the basics of ciphers.
So there's our misunderstanding. I skipped the tutorial, because I already saw your demo video. Then I clicked on the US on the map and played the first challenge called "Tutorial: Cipher salad". After that I always got the "Buy now" popup when I tried to play another challenge.
But you're right, the tutorial is playable too and it consists of the same kind of challenges, not just simple explanations how to play. So my initial statement was not correct.
I see. Well, I hope you can appreciate there are limits to what I can do – if someone skips the free challenges then is unhappy there aren't enough free challenges, I don't really know how I can fix that. If you want to go back to play the tutorial challenges, they remain available.