Evolve Inkay by using it while holding it upside down.
In Pokémon X, Y, Omega Ruby, and Alpha Sapphire: turn your Nintendo 3DS upside down when Inkay is ready to evolve.
In Pokémon Sun, Moon, Ultra Sun, and Ultra Moon: turn your Nintendo console upside down when Inkay is ready to evolve.
Trigger the evolution when Inkay is at the required level while the console is upside down.
If you don’t have the console upside down at the evolution moment, it will not evolve.