After a Life Blood Cultivator enters the Celestial World Tree, he would receive a bunch of Novice Missions from which at least one of them should be completed.
Unless the Life Blood Cultivator completes all the Novice Missions or waits until they expire, the novice period of the Life Blood Cultivator ends.
As soon as the novice period of the Life Blood Cultivator ends, he will start receiving the Primary Missions.
The Primary Missions are a series type of missions that are connected to the Life Blood Cultivator's Life Seed Ability.
Since Aeon's Life Seed Ability was a creation-type and belongs to the support category, he was given a mission that required him to sell the Life Crystals he created.
If it was an attack-type Life Seed Ability, he would be given the mission to destroy or kill something whereas a defence-type Life Seed Ability user will be given missions regarding protection.