The inputs gives proper result but button doesn't work in selenium simple program

Question

This is a simple code to add two values but whenever the code came to button it gives an error 'NoneType' object has no attribute 'click'. The values are filled up but button doesn't work.

import os
from selenium import webdriver
from selenium.webdriver.common.by import By
from selenium.webdriver.common.keys import Keys

os.environ["PATH"] += r"C:\python practice\Selenium"
driver=webdriver.Chrome()
driver.get("https://www.rapidtables.com/calc/math/Add_Calculator.html")
number1 = driver.find_element(By.NAME, "x")
number2 = driver.find_element(By.NAME, "x2")
number1.send_keys(Keys.NUMPAD1,Keys.NUMPAD5)
number2.send_keys(16)
driver.implicitly_wait(30)
btn = driver.find_element_by_css_selector('button[onclick="calc3()"]')
btn.click()