I had this same problem with my manual focus lens. I couldn't enable focus peaking. I thought it was a problem with the lens, and was about to give up on the lens. Then I switched the setting on my XT5 from back button focus, which I always use, to focus with the shutter button. And lo and behold, the focus peaking worked! I have no idea why that should make a difference, but it did. So maybe that is the problem that others are facing. Anyway, it's worth a try!