Currently there are 26 native Telephoto Prime lenses for Sony A6400 and 17 of these lenses are autofocus (AF) type.
Sony A6400 doesn't have a sensor based image stabilization system but 4 of these Telephoto Prime lenses features optical image stabilization.
Since Sony A6400 has environmental sealings (Weather Sealing), you may like to couple your camera with one of these 15 Telephoto Prime lenses with Weather Sealings.