This is happening because open_basedir restriction is applied is your server, however I tried check it from plugin editor but edit is disabled or file is not writable in your site.
Certainly something is missing in your setup there, but it is hard to tell without checking, if possible give me wp-admin and google drive login in a private reply, I shall set it up there for you.
I just tested in your installation, order_by="download_count" is working fine, actually “order_field” and “order_by” both will work fine for that short-code.
