Do you or did you bought 1 from the Treasure of the Sage? .. To use that item, you must open the Element Nature Hexagon in the Summon Cultivation Interface of the Kyukusho Deer that has the note "New Summon Feature" .. Once that Element Nature Hexagon is opened, you will see a Cultivation Hexagon inside that has the note saying "Activate Summoning". Once you clicked that, it will show a confirmation if you are willing to summon the said summon in exchange of the needed or required items ..