Are you in US? No app available on Roku. The Watch on TV is simply to sign in on Chromecast or other devices that support Just Watch. Roku does not.
@Chazz77 This is not entirely correct. It's true there isn't a Roku app but as @Sir_Launcelot said the 'Play On Tv' option will bring up a list of available devices (including Roku) and launch the streaming channel and start the film/show.
I can confirm that JustWatch on my iPad does indeed have a Play on TV option (it didn't back when this thread began), and it works with my Roku TV. Thank you @Sir_Launcelot. I haven't tried it on my Android phone yet, nor playing/casting to my old Roku box connected to my yet older TV. Yay for progress 🙂