«You do not have permission to create discussion pages, for the following reason: <Debes estar registrado para crear esta página.>»
es.wiki, being the most restrictive of Wikimedia projects, kindly reminds us how a newbie (non-autoconfirmed) feels like (for the first 50 edits, here!).
Newbies have both 'edit' and 'createtalk' right (yes I read bug 26703), so they should definitely be able to create talk pages in MediaWiki talk namespace. (Actually, they even have 'createpage'; even more confusing.)
https://es.wikipedia.org/wiki/Especial:ListaDerechosGrupos
I don't see any per-namespace restriction on https://noc.wikimedia.org/conf/highlight.php?file=InitialiseSettings.php , there are no protections in place and the page is not linked from any page so cascade protection can't be the cause either. https://es.wikipedia.org/w/index.php?title=Especial%3ALoQueEnlazaAqu%C3%AD&target=MediaWiki_discusi%C3%B3n%3AGadget-AjaxPatrolLinks.js
There are other possibilities but this seems a core bug.
Version: unspecified
Severity: normal
URL: https://es.wikipedia.org/w/index.php?title=MediaWiki_discusi%C3%B3n:Gadget-AjaxPatrolLinks.js&action=edit&uselang=en
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=36940