I has no idea how to play this game. The shooting mechanics seems some times to be working, and some it do not. And I do not know why I have to shoot.
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The how is elegantly explained in-game on the first level which acts as the tutorial. You might think the weapon is not working because you aren't choosing the right color to shoot (it depends on the "ammo" you carry). Even though its FPS you need patience and observation to progress through the levels. By shooting you illuminate the world to find your way out.
I'm sorry to hear that. Since it's a jam game the difficulty can escalate very quickly due to limited time for creating more levels or general balancing. You can always restart and try again. In the end, it has a mix of the puzzle genre and it's supposed to be hard to figure out but at least hope a bit refreshing once you find the right way :)