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\']\
You are getting a List of webElements with driver.find_elements_by_xpath(".//*[@id='UserName']") which of course not a single element and does not have send_keys() method. use find_element_by_xpath instead. Refer to this api doc.
userID = driver.find_element_by_xpath(".//*[@id='UserName']")
userID.send_keys('username')