How to fix this error: grayscaled = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY) cv2.error: /io/opencv/modules/imgproc/src/color.cpp:10638: error: (-215) scn == 3 || scn == 4 in function cvtColor?

edit

My code is :

import cv2
import numpy as np

img = cv2.imread('bookpage.jpg')
retval, threshold = cv2.threshold(img, 12, 255, cv2.THRESH_BINARY)

grayscaled = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)
retval2, threshold2 = cv2.threshold(grayscaled, 12, 255, cv2.THRESH_BINARY)
gaus = cv2.adaptiveThreshold(grayscaled, 255, cv2.ADAPTIVE_THRESH_GAUSSIAN_C, cv2.THRESH_BINARY, 115, 1)


cv2.imshow('original', img)
cv2.imshow('threshold', threshold)
cv2.imshow('threshold2', threshold2)
cv2.imshow('gaus', gaus)
cv2.waitKey(0)
cv2.destroyAllWindows()