I’m having the same issue, unable to change the name of the author of a post.
In Screen Options, “Author” shows up as an option and is checked. However, the option to change Author doesn’t appear in the Quick Edit view – everything else does (Title, Slug, Date, Password, Categories, Tags, Allow comments, Allow pings and Status)
In the full Post editor under the Document tab, I have: Status & Visibility, Permalink, Categories, Tags, Featured image, Excerpt, Discussion, and Layout. None of these items have “Author” in them.
I am using Premium with only the following plugins: GP Premium, Simple CSS
I figured it out. This is a test environment and I had only one User: Me. I guess with one user, changing an Author isn’t an option. When I added a test User, the Author field appeared in a drop-down with both users available.