ModuleNotFoundError: No module named 'sklearn.model_selection'; 'sklearn' is not a package
So after a few weeks of simply using Googles Colab I did the following.  Purchased App Cleaner and Uninstaller for Mac and got rid of all Python related apps that I do not need any more after moving to Colab and Kaggle's API.  Removed and reinstalled Python.  Ran the Idle program again and it worked!  Yay!.  The problem started while using Jupiter with Anaconda.  Just randomly appeared.  When I decided to work on it again yesterday I thought it might have been some malware on my Mac.  The reason I thought it was malware is that the Idle message was giving me the message that sklearn was not a package and it could not find train_test split - even when I asked for another sklearn module - e.g. datasets.  It always referred to train_test_split even when I did not request it.  Hence the cleanup of my Mac (everything Python related and more).  It's all good now.
I have been dealing with this much too long. I am using a MacBook Pro. This suddenly started happening when running Jupyter. I could not fix it.  I completely uninstalled anaconda (which was using Python3.8) then completely uninstalled Python3.9. I installed Python3.96.  Then went back to basics and reinstalled numpy, pandas, sklearn.  Everything seems to be there in /Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/site-packages
This command fails in IDLE and gives the error in the title of my question.
from sklearn.model_selection import train_test_split
It does not give the error in Mac's Terminal
The search path for IDLE looks correct.