Board Thread:Support Requests - Designing Your Wikia/@comment-25463843-20151202214323/@comment-4018164-20151202220103

It's in MediaWiki:Noarticletext (and MediaWiki:Noarticletext-nopermission if the user has no permission for creating that page).

If you're not sure what MediaWiki page is responsible for a certain text, add ?uselang=qqx to the end of the URL, and it will replace that text with the name of the MediaWiki page that stores that text.