Why are achievements not working ?

It’s the way the game is coded

It is not possible to gain achievements if the checksum has been changed - either by tampering with game files, or by running the game using mods that affect gameplay mechanics (graphical mods do not affect checksum), or a corrupted installation (the Ironman mode icon will change to reflect this).

Community Flavor Pack does not affect gameplay mechanics but it has to edit files in common folder (list here, where the game checksum is computed, in order to add new assets. No edit in there = no new models.

For more informations, check CK3 wiki.