If you look at the tag, it says:
There is no tag wiki for this tag … yet!
The "Save Edits" button for this tag is currently disabled, but I've checked other tag wikis and the "Save Edits" button is enabled. Entering an "Edit Summary" doesn't enable the button either.
Is this a bug, a feature for newly created tag wikis that I can't find here on Meta, or something else?