In terms of the FFFF error, can you confirm in your .ini file that you also edited second entry “MFLIX_DB_URI” in the “TEST” section and that this is also either the same as your Atlas URI or that it points to a running local mongod?
In terms of the TLS/SSL certificate errors, this appears to be a local machine issue with your machine and the certificates present. The best source of help in determining how to resolve PyMongo issues with TLS/SSL is this web page.
There is a known Python issue on Mac OS with versions of Python 3.6 or later, specifically the bug report is on this page and the resolution is to install updated root certificates.
Typically this command should resolve your issue:
open "/Applications/Python <YOUR PYTHON VERSION>/Install Certificates.command"
Please run the command listed in the previous paragraph. If that fails please refer to the PyMongo TLS/SSL web page for further assistance in terms of TLS/SSL issues.
Good luck and please let us known if this resolves your issue.