No module named cv2

[lukebuchta]

When i run the program in terminal python:

pi@raspberrypi:~/Documents/Smart-Security-Camera-master $ python main.py
Traceback (most recent call last):
File "main.py", line 1, in
import cv2

I installed opencv 4 by a manual: [https://www.pyimagesearch.com/2019/09/16/install-opencv-4-on-raspberry-pi-4-and-raspbian-buster/]

Please, do you know what i made wrong? thank you