javascript in child them

Hello everybody,

I’m developing a web site on my localhost.

I created a child theme from TwentyTwenty

Now I would like to include my own scripts in a javascript file in the child theme. I can see my javascript file on the console. If I make console.log(‘hello world’), I can see hello world on the console

On the other hand, it’s not possible to select classes. By example, if I make

console.log(document.querySelector('.site-description'))

I have null on the console

I don’t understand the problem
Do I forget something ?

Thanks and take care,

Cyril