Seito Hakari
Cheerio!
It should be intended for non-legendary or mythical that are not IV locked, Hyper Training will raise the IVs of the pokemon in battle, but does not affect HP. PS assumes it has the IVs for HP and that the pokemon is hyper trained so it can choose whatever HP you want with perfect IVs. But for pokemon like Ash-Greninja, it's IV locked so it can only use HP Ghost, but it can still be hyper trained to have 31 6 IV. Another example like Necrozma have a minimum of 3 perfect IV, so it cannot learn HP Fighting etc.You're allowed to use Hidden Power regardless of IVs. If this is due to the new Hyper Training feature added in Sun and Moon, this is not how Hyper Training works.
To quote Serebii: "The thing with Hyper Training is that the boost is superficial. The actual IVs of the Pokémon do not get increased, merely they give the appearance of having maximum. Due to this, the Pokémon's Hidden Power will remain the same as it always was, and the Pokémon, when bred, will not pass any maximum IVs as the Pokémon's IVs are still how they were originally."
Source: http://www.serebii.net/sunmoon/hypertraining.shtml