error: (-215:Assertion failed) !_src.empty() in function 'cv::cvtColor'

This error occurs when the input image is empty. In other words, the image data is not available or cannot be accessed by the function.

To resolve this error, you need to ensure that the input image is valid and can be read by the function. Check if the image file exists, and if it can be opened and read by the program. Also, ensure that the image data is not corrupted or missing.

You can also try using a different image or format to see if the error persists. Additionally, make sure that the image is in the correct color space and format for the function you are using.