I think running Forced Labor or Mercantilism. There is also a third civic which allows to keep already existing slaves but not acquire new ones I think, or maybe that was Mercantilism. I don´t really know, I rarely play farther than the Medieval Age due to my weak computer not being able to handle later eras.
No, no, I mean I can create specialists in some cities but not in others. I am running forced labor.