There is a problem in OpenCV because SIFT patent

[mnm117]

The OpenCV library is an open-source library, also the SIFT algorithm is patented, for these reasons, the SIFT algorithm does not exist in higher versions of OpenCV.
Use the following commands to execute the code completely :

pip install opencv-python==3.4.2.17
pip install opencv-contrib-python==3.4.2.17

With these commands, you can fix errors such as the following errors :

Set OPENCV_ENABLE_NONFREE CMake option and rebuild the library in function 'create'
module 'cv2.cv2' has no attribute 'SIFT_create'