wikia (Fandom)

That's because "Wikia" is a namespace here, so it can't be an interwiki prefix. For the same reason it's better to not use "Fandom". I think it used to be possible to go through Meta but the interwiki over there is no longer "local" so it doesn't redirect. I guess we could add "CentralWikia" as on m:IWM if there's an use for it.