How to send ESC key to close pop up window using Python and Selenium?

You don’t need to send keys to the element, you need to press them globally (to browser).

You can do it via Actions.

from selenium import webdriver
from selenium.webdriver.common.keys import Keys

webdriver.ActionChains(driver).send_keys(Keys.ESCAPE).perform()

You can see more info in Webdriver API – 7.2 Action Chains doc