error: (-2:Unspecified error) The function is not implemented. Rebuild the library with Windows, GTK+ 2.x or Cocoa support

I had the exact same error using yolov5, on windows 10. Rebuilding the library by typing

pip uninstall opencv-python 

then

pip install opencv-python

worked for me.

Few frustration hours later, saw this solution under the comment of the first answer by Karthik Thilakan

pip uninstall opencv-python-headless -y 

pip install opencv-python --upgrade

This worked for me in the conda environment. Thanks Karthik! 🙂

I installed another GPU and finally upgraded to Tensorflow 2 this week and suddenly, the same issue arose. I finally found my mistake and why uninstalling and reinstalling opencv works for some people. The issue is stated clearly in a text file in your opencv-python dist-packages named METADATA.

It states;

There are four different packages (see options 1, 2, 3 and 4 below) and you should SELECT ONLY ONE OF THEM. Do not install multiple different packages in the same environment.

Further, the file says that;

You should always use these packages if you do not use cv2.imshow et al. or you are using some other package (such as PyQt) than OpenCV to create your GUI.

referring to

Packages for server (headless) environments … (with) no GUI library dependencies

So, if you run;

pip list | grep opencv

and the result is more than one opencv version, you’ve likely found your problem. While an uninstall & reinstall of opencv might solve your problem, a more masterful solution is to simply uninstall the headless version as that is the one that does not care about GUIs, as it should be used in server environments.

This error is mostly with Pycharm Ide , I resolved it by changing the project interpreter None of the given solution in the internet worked for me.