Sorry about that 😔. The game has a separate publisher in Japan, so as a courtesy to them and the work they've put into the game (they helped with QA and provided the Japanese localization included in all versions of the game), I asked Valve to restrict sales of this version on Steam in Japan.
The itch version sold here is, of course, fully unrestricted and, I'd argue, is worth $15 even without a Steam code! 😛
Feel free to gift the code to a friend, but as a personal favor I ask that you do not resell it (unless you donate proceeds to charity, then I'm cool with it!).