cv2.error: OpenCV(4.5.3) C:\Users\runneradmin\AppData\Local\Temp\pip-req-build-q3d_8t8e\opencv\modules\imgproc\src\color.cpp:182: error: (-215:Assertion failed) !_src.empty() in function 'cv::cvtColor'
Try Giving Full Path.
This seems to be the path issue in windows.
I changed it to a full path like this and it worked.
filename = "D:\Sandbox\Github\opencv-project\Resources\Photos\cats.jpg"