Click the button below to get started.
I don't see any button to click, even when I scroll down:
The other answers are right; this answer on the main meta explains that tag wikis are copied from the main meta and can't be edited by anyone (even mods). However, the tagging tag on main meta has a description, and has for some time, so I'm not sure that's actually working (was tagging just created here?). In any case, the UI bug should definitely be fixed; I'll ask someone about the wiki copying.
Tag wikis can't be edited on per-site metas, AFAIK. The idea is to import them from the main Meta, but this isn't happening yet (and it wouldn't be so easy, since tag usage on metas has diverged apart from the mandatory and moderator-only tags).
I see the tag wiki creation button, but clicking it has no effect. You don't even see it because tag wikis require 20k rep to edit unsupervised and tag edit suggestions don't exist on per-site metas. We're seeing two manifestations of the same UI bug (the UI should clearly say that the tag wiki can't be edited).