I created a second Pages Library in my site as I wanted to lock down permissions to some pages. In the Advanced Settings for the document library, I allowed management of content types and selected that the document library should be a site asset library.
I then attempted to manage the content types. However, when I went to add the Wiki Page content type, I is not in the list of available content types. The type is in use in the site as I can see it in my site settings.
The challenge is that when I attempt to edit a page in the new Pages document library I created, I cannot add a web part or a web app as those button are disabled.