First do run these commands inside Terminal/CMD:
conda update anaconda-navigator
conda update navigator-updater
Then the issue for the instruction below will be resolved
For windows if you have anaconda installed, you can simply do
pip install opencv-python
or
conda install -c https://conda.binstar.org/menpo opencv
if you are on linux you can do :
pip install opencv-python
or
conda install opencv
Update:
if you use anaconda, you may simply use this as well (and hence don't need to add menpo channel):
conda install -c conda-forge opencv