I keep getting the error: Unable to locate element: {"method":"css selector","selector":".user-profile-link"}

Can you print out the HTML by using the print(driver.page_source), so it is easier for people to help you investigate and you would get more efficient support?

I believe the potential root cause would be that the portion of your code gets executed before the element that you seek finishes loading. Perhaps, try adding time.sleep(5) or if you would want to be more advance, you can use WebDriverWait

from selenium.webdriver.common.by import By
from selenium.webdriver.support.ui import WebDriverWait
from selenium.webdriver.support import expected_conditions as EC
import time


browser = webdriver.Chrome()
browser.get("https://github.com/")


browser.maximize_window()
signin_link = browser.find_element(By.PARTIAL_LINK_TEXT, "Sign in")
signin_link.click()

# add some wait time here by using either `time.sleep(5)` or WebDriverWait
# time.sleep(5)
wait = WebDriverWait(browser, 10)
wait.until(EC.visibility_of_element_located((By.CSS_SELECTOR, '#login_field')))

username_box = browser.find_element(By.ID, "login_field")
username_box.send_keys("my_github_username")
password_box = browser.find_element(By.ID, "password")
password_box.send_keys("my_github_password")
password_box.submit()

# add some wait time here by using either `time.sleep(5)` or WebDriverWait
# time.sleep(5)
wait.until(EC.visibility_of_element_located((By.CSS_SELECTOR, '.user-profile-link')))

profile_link = browser.find_element(By.CLASS_NAME, "user-profile-link")
link_label = profile_link.get_attribute("innerHTML")
assert "my_github_username" in link_label

browser.quit()