error: (-215:Assertion failed) !empty() in function 'detectMultiScale'

This error occurs when the input image is empty or null in the OpenCV function `detectMultiScale()`. The `detectMultiScale()` function is used for object detection in images or videos using the Haar Cascade classifier.

To fix this error, ensure that the input image is not empty or null. Check if the image file exists in the specified path and that the image data is loaded correctly. Also, make sure that the image data is in the expected format and size by printing the image shape.

For example, to load an image in Python using OpenCV:

import cv2

# Load the image
img = cv2.imread('path/to/image.jpg')

# Check if the image is loaded correctly
if img is None:
    print('Error: Could not load the image')
else:
    # Print the image shape
    print('Image shape:', img.shape)

    # Perform object detection using detectMultiScale()
    # ...

If the image shape is not what you expected, you may need to resize the image or convert it to the correct format using OpenCV functions such as `resize()`, `cvtColor()`, or `normalize()`.