My "next button" isn't working.

Question

I have a button that is supposed to go to diffetent text when it is pushed but it's not working. I used the same JavaScript as I did for another project and it worked so I don't know why it won't won't work for this one. When I press it the heading for the other divs for my "web vulnerabilities" section are supposed to pop up.
https://code.sololearn.com/WPMNdUMnUJ0y/?ref=app

Answer 1

https://code.sololearn.com/Wtj0mH19mktj/?ref=app

Answer 2

you are not getting element