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 

Link1 Link2

For python3.5+ check these links : Link3 , Link4

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

The Python error "ModuleNotFoundError: No module named 'cv2'" occurs for multiple reasons:

1. Not having the opencv-python package installed by running pip install opencv-python.
2. Installing the package in a different Python version than the one you're using.
3. Installing the package globally and not in your virtual environment.
4. Your IDE running an incorrect version of Python.
5. Naming your module cv2.py which would shadow the official module.
6. Declaring a variable named cv2 which would shadow the imported variable.

you can try to install using these.

# 👇️ in a virtual environment or using Python 2
pip install opencv-python

# 👇️ for python 3 (could also be pip3.10 depending on your version)
pip3 install opencv-python

# 👇️ if you get permissions error
sudo pip3 install opencv-python

# 👇️ if you don't have pip in your PATH environment variable
python -m pip install opencv-python

# 👇️ for python 3 (could also be pip3.10 depending on your version)
python3 -m pip install opencv-python

# 👇️ for Anaconda
conda install -c conda-forge opencv