OK, so let's go through what happens here: the Inheritance makes your Estates say "play the set-aside Encampment, leaving it in set-aside-land".
The Encampment tells you to get +2 Cards, +2 Actions, so you do that. It then says you can reveal a Gold or a Plunder. It then says that if you don't do the latter, to return the Encampment (not the Estates - the "this" on the Encampment refers to the Encampment itself) to the supply.
But the Encampment isn't in the play area, and so the instruction, which expects to find the Encampment in the play area, fails to move it.