selenium找不到元素的问题

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

driver = webdriver.Chrome()
driver.get("http://en.savefrom.net/")

elem = driver.find_element_by_id("sf_url")
elem.clear()
elem.send_keys("https://www.youtube.com/watch?v=0BQdUrsDegM&index=1&list=PLUY1lsOTtPeJNBuSweXS9pcSKbP4mr32S")
elem.send_keys(Keys.RETURN)

time.sleep(5)
elem.find_element_by_id('sf_frame')

错误:

D:\Anaconda2\python.exe C:/Users/Administrator/PycharmProjects/untitled3/selenium-test.py
Traceback (most recent call last):
  File "C:/Users/Administrator/PycharmProjects/untitled3/selenium-test.py", line 14, in <module>
    elem.find_element_by_id('sf_frame')
  File "D:\Anaconda2\lib\site-packages\selenium\webdriver\remote\webelement.py", line 138, in find_element_by_id
    return self.find_element(by=By.ID, value=id_)
  File "D:\Anaconda2\lib\site-packages\selenium\webdriver\remote\webelement.py", line 478, in find_element
    {"using": by, "value": value})['value']
  File "D:\Anaconda2\lib\site-packages\selenium\webdriver\remote\webelement.py", line 461, in _execute
    return self._parent.execute(command, params)
  File "D:\Anaconda2\lib\site-packages\selenium\webdriver\remote\webdriver.py", line 236, in execute
    self.error_handler.check_response(response)
  File "D:\Anaconda2\lib\site-packages\selenium\webdriver\remote\errorhandler.py", line 192, in check_response
    raise exception_class(message, screen, stacktrace)
selenium.common.exceptions.NoSuchElementException: Message: no such element
  (Session info: chrome=51.0.2704.103)
  (Driver info: chromedriver=2.7.236900,platform=Windows NT 6.1 SP1 x86)
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