I have the item "Angelic Aeanoid Roott" in my inventory. But when I go to the garden, there is no option to plant the seed. Why?

it's not a seed. Aeanoid Seed is a seed. There are pods that are seeds, but most other things need to say that they are seeds in order to act as seeds.

You can double check which is which on the fansite SubetaLodge, each item will have Notes that you can search.

If you do plant and water the Aeanoid Seed be aware that it will attack your active pet when it reaches maturity.