I tried to redeem the Steam key, but Steam gave me an error that it can't be redeemed in my country (Japan.)
Could you look into any region locks?

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!).