Retrieving HTML element object in Python web driver

I am trying to automate downloading of some csv files from my Splunk account. We don't have API access unfortunately.

So I thought of using Selenium Python drivers for this.

I installed Selenium and put the chrome driver in the right path.

Now the first page I want to access is the login page of Splunk as below

Here is my initial code in Python using Selenium

import selenium
from selenium import webdriver

PATH= "https://splunk.com/account/login"

driver = webdriver.Chrome()
# Open the website
driver.get(path)


username_box = driver.find_element_by_name('username')

# Get the Password box object 
password_box = driver.find_element_by_name('password')


# Send id information for login
username_box.send_keys(USERNAME)
password_box.send_keys(PASSWORD)

# Click on the sign in button

sign_in_button=driver.find_element_by_link_text('Sign in')
sign_in_button.click()

It works perfectly fine until the sign_in_button driver. So it is able to open the page, enter the user name and password from the user_name and password_element, but when it comes to retrieving the Sign In element object, it's not able to retrieve the object and throws the error:

NoSuchElementException: Message: no such element: Unable to locate element: {"method":"link text","selector":"Sign in"}
  (Session info: chrome=71.0.3578.98)

Here is the HTML of the corresponding Sign In button when I do Inspect on Chrome

So as you see it has only the class and text to it. No name and Id attribute.

So I tried first

driver.find_element_by_link_text('Sign in')

and then

driver.find_element_by_class_name('splButton-primary btn') 

but is not able to retrieve the Sign in button object.

Because of which it's not able to click and login.

Could someone help what is going wrong here with respect to retrieving the Sign in Button?

Thanks