Slitherine

In the Province of Sicily I have stud farm, stables and Castrum Cataphractae but cannot build heavy cavalry or cataphracts. Any idea what the issue is? Missing the stable master (I don't think so)? I am playing Carthage

Not all nations have access to all units, and for Carthage they don't have access to the units your mention, sorry. Well, they have mercenary cavalry though, but no native Carthaginian cavalry beyond LC.