Python 2.7 is not supported anymore in opencv-python-18.104.22.168 the support was dropped since 22.214.171.124,
The workaround I found was to install opencv-python version 126.96.36.199 (which is the latest supported for Python 2.7, see this for all releases) like this:
pip2 install opencv-python==188.8.131.52
Hope this helps!!
If you need to learn more about Python, It's recommended to join Python Training today.