Open CV syntax in google colab - opencv

Question

In the following code, I was trying to print the normalized image, which worked well in the google Colab environment. But as soon as I converted that image matrix to a flattened matrix it simply resulted in an error.
import cv2
import numpy as np
import matplotlib.pyplot as plt
from google.colab.patches import cv2_imshow
%matplotlib inline
import os
vertical = cv2.imread("vertical (1).PNG", cv2.IMREAD_GRAYSCALE)
horizontal = cv2.imread("horizontal (1).PNG", cv2.IMREAD_GRAYSCALE)
vertical = vertical/255
horizontal = horizontal/255
print(vertical)
print(horizontal)
vertical_flattened = vertical.flatten()
horizontal_flattened = horizontal.flatten()
print(vertical_flattened)
print(horizontal_flattened)
plt.rcParams["figure.figsize"]=(20,10)
plt.subplot(2,3,1)
plt.title('Vertical Image')
plt.imshow(vertical, cmap = 'gray')
plt.subplot(2,3,4)
plt.title('Vertical_flattened Image')
plt.imshow(vertical_flattened, cmap = 'gray')
plt.subplot(2,3,2)
plt.title('Horizontal Image')
plt.imshow(horizontal, cmap = 'gray')
cv2.waitKey(0)
The error details are as follows :
TypeError Traceback (most recent call last)
<ipython-input-9-c30222345864> in <module>
25 plt.subplot(2,3,4)
26 plt.title('Vertical_flattened Image')
---> 27 plt.imshow(vertical_flattened, cmap = 'gray')
28
29 plt.subplot(2,3,2)
TypeError: Invalid shape (61746,) for image data
What should I do to print the flattened image? Thanks in advance.