As an admin it works.
Can you please try the following: as an Admin (EDIT: where you can start robotmk), start a Python shell:
❯ python
Python 3.8.9 (default, Oct 26 2021, 07:25:53)
[Clang 13.0.0 (clang-1300.0.29.30)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import six
>>> import inspect
>>> inspect.getfile(six)
'/.............../lib/python3.8/site-packages/six.py' # WHAT IS THE LOCATION OF six ON YOUR MACHINE?
>>>
I know that when modules were installed as normal user, another following installation attempt in an Admin cmd will report that the module is “already installed”.
You are able to use the module from this admin cmd (because it can also load the module from your user profile), but NT AUTHORITY SYSTEM (where the agent runs under) does not have access to that profile.