I think I’ve found a small typo in the docs here
The doc says
pip install -U pip
pip install rasa-x==0.35.0 --extra-index-url [https://pypi.rasa.com/simple](https://pypi.rasa.com/simple)
however, the second command should be
pip install rasa-x==0.35.0 --extra-index-url https://pypi.rasa.com/simple