I mean this game is created on the Raspberry Pi device.
No, it's not. The Raspberry Pi version use exactly the same code than any other version, and so the same format for the RPA archives.
It can't be opened because the authors of the game tweaked Ren'py to put a small alteration of the RPA format. They changed it's signature to "RPI-3" and added two insignificant bytes before the key used to encrypt the index. Nothing that can't be reverted, but the problem with RPA alteration is that they need to edit unrpa (or other) code each time.
@CobraPL , attached you'll find a version of unrpa that should open those RPA files (tested only with the previous version of the game, I greatly dislike games that use an installer). It also works with regular RPA for convenience, but I don't recommend to use it with other games than this one.