I had the same problem.
I deinstalled the addon with curseforge, downloaded the addon from the Buffed website and extracted everything in the addon folder manually.
Now it shows up in the Interface -> Addon Section and can be shown with /omen show
Thank you, this worked.
Little sidenote: couldn't post a comment on the addon site (https://www.curseforge.com/wow/addons/omen-threat-meter), gave me an error every time, tried for about a week before googling and coming here.