I was able to fix it by replacing

      title: "Glance Image List"
      scenario:
        GlanceImages.list_images: 
          detailed: True

with

      title: "Glance Image List"
      scenario:
        GlanceImages.list_images: {} 


But now I am getting tons of multiprocessing errors:


2020-11-06 11:52:48.828 35147 ERROR rally.task.engine [-] Unexpected exception during the workload execution: TypeError: 'NoneType' object is not callable
2020-11-06 11:52:48.828 35147 ERROR rally.task.engine Traceback (most recent call last):
2020-11-06 11:52:48.828 35147 ERROR rally.task.engine   File "/Users/i530566/devel/SAP/rally/.venv/lib/python3.8/site-packages/rally/task/engine.py", line 533, in _run_workload
2020-11-06 11:52:48.828 35147 ERROR rally.task.engine     runner_obj.run(workload["name"], context_obj,
2020-11-06 11:52:48.828 35147 ERROR rally.task.engine   File "/Users/i530566/devel/SAP/rally/.venv/lib/python3.8/site-packages/rally/task/runner.py", line 166, in run
2020-11-06 11:52:48.828 35147 ERROR rally.task.engine     self._run_scenario(scenario_plugin, "run", context, args)
2020-11-06 11:52:48.828 35147 ERROR rally.task.engine   File "/Users/i530566/devel/SAP/rally/.venv/lib/python3.8/site-packages/rally/plugins/task/runners/constant.py", line 244, in _run_scenario
2020-11-06 11:52:48.828 35147 ERROR rally.task.engine     process_pool = self._create_process_pool(
2020-11-06 11:52:48.828 35147 ERROR rally.task.engine   File "/Users/i530566/devel/SAP/rally/.venv/lib/python3.8/site-packages/rally/task/runner.py", line 192, in _create_process_pool
2020-11-06 11:52:48.828 35147 ERROR rally.task.engine     process.start()
2020-11-06 11:52:48.828 35147 ERROR rally.task.engine   File "/Users/i530566/.pyenv/versions/3.8.5/lib/python3.8/multiprocessing/process.py", line 121, in start
2020-11-06 11:52:48.828 35147 ERROR rally.task.engine     self._popen = self._Popen(self)
2020-11-06 11:52:48.828 35147 ERROR rally.task.engine   File "/Users/i530566/.pyenv/versions/3.8.5/lib/python3.8/multiprocessing/context.py", line 224, in _Popen
2020-11-06 11:52:48.828 35147 ERROR rally.task.engine     return _default_context.get_context().Process._Popen(process_obj)
2020-11-06 11:52:48.828 35147 ERROR rally.task.engine   File "/Users/i530566/.pyenv/versions/3.8.5/lib/python3.8/multiprocessing/context.py", line 284, in _Popen
2020-11-06 11:52:48.828 35147 ERROR rally.task.engine     return Popen(process_obj)
2020-11-06 11:52:48.828 35147 ERROR rally.task.engine   File "/Users/i530566/.pyenv/versions/3.8.5/lib/python3.8/multiprocessing/popen_spawn_posix.py", line 32, in __init__
2020-11-06 11:52:48.828 35147 ERROR rally.task.engine     super().__init__(process_obj)
2020-11-06 11:52:48.828 35147 ERROR rally.task.engine   File "/Users/i530566/.pyenv/versions/3.8.5/lib/python3.8/multiprocessing/popen_fork.py", line 19, in __init__
2020-11-06 11:52:48.828 35147 ERROR rally.task.engine     self._launch(process_obj)
2020-11-06 11:52:48.828 35147 ERROR rally.task.engine   File "/Users/i530566/.pyenv/versions/3.8.5/lib/python3.8/multiprocessing/popen_spawn_posix.py", line 47, in _launch
2020-11-06 11:52:48.828 35147 ERROR rally.task.engine     reduction.dump(process_obj, fp)
2020-11-06 11:52:48.828 35147 ERROR rally.task.engine   File "/Users/i530566/.pyenv/versions/3.8.5/lib/python3.8/multiprocessing/reduction.py", line 60, in dump
2020-11-06 11:52:48.828 35147 ERROR rally.task.engine     ForkingPickler(file, protocol).dump(obj)
2020-11-06 11:52:48.828 35147 ERROR rally.task.engine TypeError: 'NoneType' object is not callable
2020-11-06 11:52:48.828 35147 ERROR rally.task.engine
2020-11-06 11:52:48.828 35147 INFO rally.task.engine [-] Finished workload 1/1 of subtask 8  (completed 10 of 10 in general).
2020-11-06 11:52:48.837 35147 INFO rally.task.engine [-] Task 95c58d72-1bb6-49ba-a569-2a8da5744737 | Completed: Running task.