Face tagging

What face tagging functionality are you referring to?

To be clear, Photostructure does not have a way to add tags currently. It can read and display tags that you’ve applied, but you cannot add new tags.

Adding tags, and face tagging specifically, are outstanding feature requests. You can read more about them in the following posts:

edit: Photostructure WILL pick up new tags that you add. So, if you add a tag in another program, Photostructure will pick that up on the next sync.