Helo,
my xpath does validate in firePath but when I try to send _key I get an error.
userID = driver.find_elements_by_xpath(\".//*[@id=\'UserName\']\
Please replace your code with the following code because you like addressing the first component of your list, not whole list.
userID = driver.find_elements_by_ID('username')[0]
userID.send_keys('username')
or delete "s" from find_elements_by_ID such as follow:
userID = driver.find_element_by_ID('username')
userID.send_keys('username')