Win10 here, but I still see the Save button in PR 15.1. That is true even if I have not added a filename. And I tried creating the file in a location for which I do not have permission. The Save button is okay; I just get a prompt saying I don't have permission.
Did you name it and Save was still disabled? Save is disabled on Mac, as the Save As field is empty by default. Once you type something in the Save As field, then the Save button should enable. Windows will have Save enabled, even if the field is empty, but won't actually save until you type something in the field.
I know it's bit wonky, but the dialog is based on OS functionality (which is why Mac and Windows is a bit different). We have an open issue to add a default name for Save As, to avoid this issue.