Which Pokémon can't be shiny in Sun/Moon?
Solution 1:
According to Serebii this is the list of shiny-locked Pokemon in Sun/Moon:
- Tapu Koko (With the exception of the event Tapu)
- Tapu Lele
- Tapu Bulu
- Tapu Fini
- Cosmog
- Solgaleo
- Lunala
- Nihilego
- Buzzwole
- Pheromosa
- Xurkitree
- Celesteela
- Kartana
- Guzzlord
- Necrozma
- Zygarde
Probably also Magearna and Marshadow are shiny-locked like other Event-only Legendaries, they are listed in a different table.
Solution 2:
It would appear that Solgaleo, Lunala, and the Ultrabeasts are shiny locked
Secondary sources here and here
Evidence that Type: Null is not shiny locked