I had the same problem. Here is how I solved it.
I have downloaded Anaconda python version 3.7. I already had Python 3.6.7 on my system.
I’d get this error if I did:
pip install jupyter_contrib_nbextensions
To solve it, I did:
python3.7 -m pip install jupyter_contrib_nbextensions
I have not tried this, but this could solve your problem too:
conda install -c conda-forge jupyter_nbextensions_configurator
So I guess the problem is because of there being multiple versions of Python on your system. The one which Anaconda uses and the one which the “normal pip” uses are probably different.