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You having typo comma cv2, rectangle. Your code doesn't do anything cv2.waitKey.

import cv2

cap = cv2.VideoCapture(0)

cap.set(3,640)
cap.set(4,380)

while cap.isOpened():
    ret, frame = cap.read()
    if not ret:
        break

    cut = cv2.rectangle(frame, (220, 200), (450, 400), (255,0,0), 2)
    cv2.imshow('cut', cut)

    key = cv2.waitKey(10)
    if key == 27 or key == 113:
        break

cap.release()
cv2.destroyAllWindows()