Okay, I fixed it! I for some reason had cookies from third party websites (or whatever) blocked. I guess I never played a web game where you could save, so the error never occured for me till now.
As for the actual game, I can't really give feedback yet. I haven't even left the first room/scene yet because after realizing I couldn't save, I didn't continue to play cuz I would've lost the progress anyway..
But now that I can save, I'll be sure to leave feedback when I've gathered enough to make an actual opinion. For now, all I can say is that it looks pretty nice.
EDIT: Ok, you reeeally need to change the fighting system. The fact that you can't open your menu/inventory during a "fight" is bad enough, but that you can't even leave the zone during one is even worse. Like, besides trying to progress through the map, the main reason why you would want to leave a zone is to get out of a fight, heal up, etc. If you can't do that, you might aswell give me the "Game Over" screen instantly. Also, putting "fight" in quotation marks because.. they're not fights. Please give the zombies range so that once you're out of the range, you're not in a fight with them anymore. I'm speedrunning through a map and didn't realize I passed one of the zombies & it noticed me. So now, I'm tryna exit the zone but I can't because I'm in a "fight" with a zombie on the other side of the zone and you're telling me I need to walk through the whole map again, try to figure out which one of them looks like they're walking towards me instead of just walking randomly, and then kill them without getting any other zombies to notice me? Nah man. That's really bad game design.
And a side thing, but not that important if you make it so we can open inventory/leave zones during fights: the zombies are way too strong way too fast in my opinion. Like, third/fourth zone and you're already having trouble because the knife just sucks too much and you can't get a gun because it's too pricey for now.