The correct answer is - temperate rainforests of the Olympic Peninsula.
The Pacific northwest tree octopus is a fictional animal, thus it does not exist in the present, nor there is any proof that such a creature existed in the past, though there's every chance that it can evolve in the future.
According to the description of this fictional octopus species, unlike the octopuses we know, it is actually amphibious. This basically means that this octopus is able to live in the water, but also be terrestrial. It has developed from the octopuses in the East Pacific, and it started mostly to live on land, or rather on trees. It has used its eight tentacles in order to be able to have a perfect tool and easy arboreal life, swinging from one branch to another, being able to cover longer distances very easily, and manage to hunt with relative ease.