How to create Axis-aligned bounding boxes

I am trying to get axis-aligned bounding box where the original algorithm used rotated bounding boxes. corresponding lines are --
if len(contours) != 0 and np.max(cnt_area) > 100:
contour = contours[np.argmax(cnt_area)] # use max area polygon
polygon = contour.reshape(-1, 2)
pbox = cv2.boundingRect(polygon) # Min Max Rectangle
#pbox = cv2.boxPoints(cv2.minAreaRect(polygon))
box_in_img = pbox
state['ploygon'] = rbox_in_img if mask_enable else []
location = state['ploygon'].flatten()

But getting the following error:


Then if I delete the flatten() function in demo.py, I do not get errors but bounding boxes are not correct.

Please give me some suggestion to solve this issue.