Just the reward Petilil(s) aren't enough to evolve Petilil ... They share a pool with the other evolution items, but a Sun Stone will eventually pop up with enough perseverance.

Sunkern to Sunflora Evolution cost - 1 Sun Stone and 50 Sunkern Candy. The first Pokémon to make use of a Sun Stone in several generations is Gen 5's Petilil, allowing it to evolve into Lilligant ...