This script check whether your web camera successfully starts and how to work.

webcam.py

import time

import cv2
import numpy as np


def webcam():
    video_capture = cv2.VideoCapture(0)
    time_keep = time.time()
    while True:
        _, frame = video_capture.read()

        fps = 1 / (time.time() - time_keep)
        time_keep = time.time()
        cv2.putText(
            frame,
            f'{fps:.2f} FPS',
            (10, 30),
            cv2.FONT_HERSHEY_SIMPLEX,
            0.75,
            (0, 255, 0),
            thickness=1
        )
        cv2.imshow('Res', frame)

        # stop by 'q' command
        if cv2.waitKey(1) & 0xFF == ord('q'):
            break

    video_capture.release()
    cv2.destroyAllWindows()


if __name__ == '__main__':
    webcam()