It’s because when you execute the script the element does not exist in the page. Any script in the head that tries to access anything in the body right away will throw errors.
You need to wait for the element to exist.
It can be done in different way.
You can use a mutationObserver, this post has an example for a similar problem.
Another option is to execute the code in response to a user action, when you are sure the corresponding element is oresent in the page.
The best option is to use a custom component to integrate this logic.
On a side note, element.click() is different from the click method in jquery. The former is used to fire a click event, the latter is used to bind a event handler.