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I solved problem. Look in line 29:

#!/usr/bin/python37
#OpenCV 4.3.0, Raspberry pi3B/+,4b/4g/8g, Thonny 3.7.3
#Date: 29th June, 2020

import cv2
import numpy as np

img1 = cv2.imread('ralph.jpg')
img2 = cv2.imread('mainlogo.png')

# I want to put logo on top-left corner, So I create a ROI
rows,cols,channels = img2.shape

roi = img1[0:rows, 0:cols]

# Now create a mask of logo and create its inverse mask also
img2gray = cv2.cvtColor(img2, cv2.COLOR_BGR2GRAY)
ret, mask = cv2.threshold(img2gray, 10, 255, cv2.THRESH_BINARY)
mask_inv = cv2.bitwise_not(mask)

# Now black-out the area of logo in ROI
img1_bg = cv2.bitwise_and(roi, roi, mask=mask_inv)

# Take only region of logo from logo image.
img2_fg = cv2.bitwise_and(img2, img2, mask=mask)

# Put logo in ROI and modify the main image
dst = cv2.add(img1_bg, img2_fg)
img1[450:rows+450, 450:cols+450] = dst

cv2.imshow('res', img1)
cv2.waitKey(0)
cv2.destroyAllWindows()

Output:

I solved problem. Used double for both numbers not unique. For instance for row is 450 and 450, but not 450 and 150. Otherwise, you will get an error. Look in line 29:

#!/usr/bin/python37
#OpenCV 4.3.0, Raspberry pi3B/+,4b/4g/8g, pi3B/+, 4b/4g/8g, Thonny 3.7.3
#Date: 29th June, 2020

import cv2
import numpy as np

img1 = cv2.imread('ralph.jpg')
img2 = cv2.imread('mainlogo.png')

# I want to put logo on top-left corner, So I create a ROI
rows,cols,channels = img2.shape

roi = img1[0:rows, 0:cols]

# Now create a mask of logo and create its inverse mask also
img2gray = cv2.cvtColor(img2, cv2.COLOR_BGR2GRAY)
ret, mask = cv2.threshold(img2gray, 10, 255, cv2.THRESH_BINARY)
mask_inv = cv2.bitwise_not(mask)

# Now black-out the area of logo in ROI
img1_bg = cv2.bitwise_and(roi, roi, mask=mask_inv)

# Take only region of logo from logo image.
img2_fg = cv2.bitwise_and(img2, img2, mask=mask)

# Put logo in ROI and modify the main image
dst = cv2.add(img1_bg, img2_fg)
img1[450:rows+450, 450:cols+450] = dst

cv2.imshow('res', img1)
cv2.waitKey(0)
cv2.destroyAllWindows()

Output:

#!/usr/bin/env python35
#OpenCV 4.3.0, Raspberry pi3B/+, 4b/4g/8g, Thonny 3.7.3
#Date: 1st July, 2020

import cv2
import numpy as np

img1 = cv2.imread('ralph.jpg')
overlay_img1 = np.ones(img1.shape,np.uint8)*255

img2 = cv2.imread('mainlogo.png')
rows,cols,channels = img2.shape
overlay_img1[450:rows+450, 450:cols+450 ] = img2

img2gray = cv2.cvtColor(overlay_img1,cv2.COLOR_BGR2GRAY)
ret, mask = cv2.threshold(img2gray,220,55,cv2.THRESH_BINARY_INV)
mask_inv = cv2.bitwise_not(mask)
temp1 = cv2.bitwise_and(img1,img1,mask = mask_inv)
temp2 = cv2.bitwise_and(overlay_img1,overlay_img1, mask = mask)

result = cv2.add(temp1,temp2)
cv2.imshow("Result",result)
cv2.imwrite("Result.jpg",result)
cv2.waitKey(0)
cv2.destroyAllWindows()