Q: Force touch to look up doesn't work on Preview app

FWIW, Force Touch or Three Finger Tap Look Up does not work in Preview on Sierra 10.12. Hopefully it will be fixed in 10.12.1.

I should add, you can select a word in Preview, then right/control click on the word and choose Look Up from the drop down menu as a workaround.

You don't even need to select a word. Just have the cursor hovering over it, followed by either a two-finger tap, or right mouse button click, to get at the Lookup on the contextual menu.

Poor choice of wording on my part. Select as in choose, pick, etc.

Yes. That doesn't work in Sierra. As I said, hopefully it will be fixed in the next update.