You must reach Rank 5 Renown with the Dream Wardens to unlock Elianna
You will need to complete activities in the dream to harness energy for Elianna to give you a Dream Infusion. There will be no notification to let you know you've reached 100% on the bar, except she will have a blue ! over her head. Any harnessed energy you would have gotten beyond 100% does not carry over into the next bar.
You will need to summon a pet with the same likeness as the one you are purchasing to officially unlock it for purchase: