Pystan is installed but No module named 'pystan'

[Zman67]

Reason for this error can be found on the internet:

https://stackoverflow.com/questions/68826506/modulenotfounderror-no-module-named-pystan

However the mentioned workarround doesn't work for me.

pip install "pystan<3.0.0" cannot be installed as well.

So kind request to change import pystan into import stan