What you’re seeing here is a limitation of our existing folder mechanism. Most doc permissions are stored in GDrive today, but due to technical limitations, folder sharing is not - that lives exclusively in Coda. When you have access to a doc via folder sharing only, GDrive does not think you have permission to the doc or permission to edit the permissions on the doc…therefore you won’t be able to change the sharing permissions of the doc via the Share button.
We are looking into some workarounds for this, but don’t have a great solution at the moment. We’re also working on the ability to have Coda docs be completely separate from GDrive to eliminate cases like this.