at the
Installation: 4. Install ns3gym located in src/opengym/model/ns3gym (Python3 required)
there is no opengym under the src. do you mean ns-3.XX/contrib/opengym/model/ns3gym instead?
You need to clone this repo and use the ns-3 that comes with, which is 3.29 I believe. You can't install ns3-gym with an existing ns3 installation (not that I know of). The instruction uses setup.py to install the python module located here.
at the Installation: 4. Install ns3gym located in src/opengym/model/ns3gym (Python3 required) there is no opengym under the src. do you mean ns-3.XX/contrib/opengym/model/ns3gym instead?