It is not working because Button "Log Out" needs some loading time to be shown in website.  
So use this it will definitely work for all. Sleeping time should be given to load text inside that button after clicking the button and before clicking on log out


From selenium import webdriver  
import time:

    User_Name = input(">")
    Pass_Word = input(">")
    if len(User_Name) > 1 and len(Pass_Word) > 1:
        browser = webdriver.Chrome()
        browser.get("https://facebook.com")
        sign_in = browser.find_element_by_link_text("Log In")
        sign_in.click()
        username = browser.find_element_by_id("email")
        username.send_keys(User_Name)
        password = browser.find_element_by_id("pass")
        password.send_keys(Pass_Word)
        password.submit()
        button = browser.find_element_by_id("userNavigationLabel")
        button.click()
        time.sleep(20)
        l_og = browser.find_element_by_link_text("Log Out")
        l_og.click()
        browser.close()
    else:
        raise KeyError