Gracidea is a special flower that was introduced in Pokemon Diamond and Pearl, and it is not available in the Alola region. It is needed to evolve certain Pokemon like Sneasler, which evolves from Sneasel by using a Gracidea.
If you want to evolve Sneasel into Sneasler, you'll have to trade a Gracidea from a Pokemon Diamond, Pearl, Platinum, or Brilliant Diamond/Shining Pearl game.