Also you are calling functions like document.getElementById in the <head> script, but the element will not exist at this point. You would need to call at a later point, like using the On Scene Load handler to Run JavaScript….
However making these two changes (the other being adding jquery), it doesn’t appear to work; as far as I can tell the parallax.js library you included is creating a <body> element for some reason and that is interfering with the page. You may want to try a different library or lazily loading.
There’s probably other solutions on the forums for what you’re trying to do; a quick search shows this one by @MarkHunte: Mouse Move Parallax