But it didn't said anything about placing an earth plate. There's nothing in there
You need to have earth plate in your inventory to plate it. In this picture, if you have elemental plates in your inventory, there will have options to place it.

Do you have any half plate yet? There are 2 half plate to collect
> Left-half from 2nd lv. Dungeon at 2,3. Then, go back to Rat king and talk about a half plate with Tao.
> Right-half from talking with Rat king about half plate.
> Full Air plate from combine Left and Right half plate. Check on inventory>>Misc.>> click 'use' at any half plate to combine.