cpp:1689: error: (-215:Assertion failed) !empty() in function 'cv::CascadeClassifier::detectMultiScale'

This error message is generated by the OpenCV library when the detectMultiScale method of the CascadeClassifier class is called with an empty input image. The method is used to detect objects in an image using a trained classifier.

To fix this error, make sure that the input image is not empty and has valid data. Check the path to the input image file and make sure it is correct. You can also check the image dimensions and make sure they are not zero.

If the input image is valid, the error could be caused by an issue in the classifier or the parameters used in the detectMultiScale method. Try using a different classifier or adjusting the parameters to see if it resolves the issue.