I recently moved a Lugia from my AS to my Pokemon Sun and now when I try to move it back it won't let me. Why?

Question

Thanks in advance.

Answer 1

Any Pokémon caught in or transferred to the Generation VII games cannot be transferred to the Generation VI games. Such Pokémon are outlined in a white square when using a Generation VI game with Pokémon Bank.

This is to prevent complications involving transferring new things to old games, where those new things don't exist.
source

Comments

Which is extremely lazy, seeing as trading directly between Gen I and Gen II back in the day was no problem, but trading INdirectly between Gen VI and Gen VII through Bank is too hard, despite all the tech available.