Jupyter command `jupyter-contrib` not found.

The `jupyter-contrib` command is not a built-in command in Jupyter. It is a package that needs to be installed separately. You can install it using pip by running the following command in your terminal:

pip install jupyter_contrib_nbextensions

After installing the package, you can enable the extensions by running the following command:

jupyter contrib nbextension install --user

This will install a set of useful extensions that can enhance your Jupyter experience. You can then enable individual extensions by going to the Jupyter Notebook dashboard and selecting the `Nbextensions` tab.