/execute with fill and testforblock

when I uesing a mincraft command block i did /execute @a ~~~ fill _{10~10~10 ~-1}-1~-1 air 0 replace Lave. but when I used it it said the fill command does not work with /execute. wan I used it would fill some parts of lave and some parts it would just stay and I tested it may blocks but it did the same thing. another thing was /execute @e[type=armor_stand] ~~~ testforblock ~10~10~10 obsidian but it said the same thing as fill. but this time it did not work at all. I did try it on me but it still did't work. sry about not having a picture.