i can help you in game if you want just ping me on my discrd BDO#4442
im not sure exactly what you mean by filling an area in but you can use the wooden axe to select a cuboid shape and the //set {block} to make it a certain block or many blocks. if you are trying to fill a specfic outline shape you stand inside the selection and type //fill block radius depth.
idk if thats wat ur tryna do if that doesnt help just dm me.