This one is a classic: the SharePoint Publishing feature in a Publishing Site creates a document library by the name of Pages; this is where you can store your publishing pages.
A common request is to have more of these document libraries, that is because we cannot create publishing pages anywhere else. The problem is, it is unsupported, God knows why!
What you can do is rename the document library to something else. SharePoint will look for the library id in the site’s property bag, under __PagesListId, so only need to update this value accordingly:
web.AllProperties["__PagesListId"] = newDocLibId.ToString();
Now, there are some solutions over the Internet that claim to go around this limitation, but none of them is supported, so proceed with care!