Error 215 is a sign that the image is not in a proper format fot the given color conversion.

scn == 3 || scn == 4 suggests that img is not a 3-channel or a 4-channel image, as required for BGR2GRAY.

Please add

print img.shape and see if the 3rd value is 1, instead of 3. This could be the reason for the error.

Error 215 is a sign that the image is not in a proper format fot the given color conversion.

scn == 3 || scn == 4 suggests that img is not a 3-channel or a 4-channel image, as required for BGR2GRAY.

Please add

print img.shape and see if the 3rd value is 1, instead of 3. This could be the reason for the error.