i don’t know how much clearer i can make this statement. your game’s browser-embed is set, in the itch.io settings, to be 1920x1080. If you want a 16:9 aspect ratio, set it to 640x360, not 1920x1080. Why? Because 1366x768 is an extremely common resolution and anyone playing on such a display will have to scroll horizontally in order to reach the fullscreen button in order for your game to be fully visible.
After doing so the game will play fine, but doing so will be difficult, because the fullscreen button only exists after they have pressed “Run Game” which disables scrolling.
tl;dr: 640x360, not 1920x1080