The web browser. I'm using Chrome.
This happens to me alot for other browser games aswell, so its not really your game that is causing the problem.
Viewing post in Deltarune: Lightners Live Plus comments
Still doesn't work but, I found out the problem, but I'm not sure if there's really a fix.
Turns out its because I don't have WebGL2. It isn't easy as just "Enabling it". It literally does not show up anywhere.
I can just install the game so it isn't really a big deal.
The error code was: "The following features required to run Godot projects on the Web are missing:
WebGL2 - Check web browser configuration and hardware support".
(The problem wasn't graphic acceleration in Chrome settings.)