From the module description page, it looks like D11 support exist
However find the php error log and see what exactly is the error message
Site encountered an expected error is a generic
Once you find the actual error message relating to aggregator module search for the same in the issue queue of that module whether it is an existing one. If not create an issue also posting the actual error screenshot here might give an idea too.
Maybe this should help on flexslider
https://youtu.be/Jg3zdKt5Uxk?si=SXZSFMxB1AdAWeWs
It's all about installing the libraries properly and getting it loaded
See if this module is helpful
drupal learn → created an issue.
https://www.drupal.org/project/tfa →
If you're looking for your own solution, probably this could work
Use the Drupal mail service to send a six digit code to the user upon login using login hook and if the password entered is correct then redirect them to a custom page or form where you validate the generated six digit code sent to that particular user email(you've have to save the code after generated maybe in the current session variable) and if it matches then let the user in and purge the generated code from the session variable.
I'm no expert, however this could probably work.