When you first open Excel There is a green window on the left that is titled Recent. This won't work there. That would be way too easy. You can only pin files here not folders. Select a file and open it. Either click the open or open recent icons on the
top ( I added them to the quick access toolbar) or select File / Open and browse. Here you should be able to right click a folder and select "Pin to quick access". In the little window on the left you can scroll up and see the Star with the Quick Access header.
The one benefit of doing it this way is that the Quick Access header is there any time you browse for a file. Open Windows Explorer or File Explorer and it's there too.
Hope this explains it better.
John