I'm not sure if that can help:
First take a look if Zorin appindicator is enabled. If not, enable it and check if you can see the tray icon when you launch nordvpn.
You could try to disable the Zorin Appindicator gnome extension and try it with this gnome extension:
First install extension manager. Disable the Zorin Appindicator at tab installed. Then browse for the extension (choose shell version 43 for Zorin 17 and shell version 46 for Zorin 18) and install it.