Why it does not work ?

Question

https://code.sololearn.com/W0TVXAc3QGy1/?ref=app

Answer 1

because you try to load jQuery from sololearn servers ^^

replace your relative url in src attribute with:

https://ajax.googleapis.com/ajax/libs/jquery/3.6.0/jquery.min.js

Answer 2

Change $('button'), with
$('#demo') and it will work