Trying to Get Lights On My Button To Turn On

JavaScript
18

Unfortunately it wont work in GitHub! https://mtzioncode.github.io :frowning: :frowning:
It says, “Cannot read property ‘addEventListener’ of null” and shows a squiggly red line underneath this showcase.addEventListener("mouseover", function(){ (even though it works in Code Pen)!!
Also, my other question is here: https://forum.freecodecamp.org/t/how-to-enable-javascript-blinking-light-for-hover/199828/1