The package python-openzwave from https://pypi.org/project/python-openzwave/ is needed for ZWave Plugin to work.
Unfortunately this package won't install with Python 3.9 e.g. on Debian bullseye.
The Package is currently unmaintaned and does not get any fixes. It is unlikely that existing problems will be fixed in nearer future.
There are other projects like Eventghost or Homeassistant that seem to face the same problems. They seem to have a solution, see at blacklight/platypush#128 @kdschlosser seems to have developed a solution.
I don't know if https://pypi.org/project/homeassistant-pyozw/ can provide a solution. However I do not have hardware an can not test or develop another integration. So if we do not find a solution I vote for removing the plugin from the plugin repo so that Travis CI tests won't fail any more.