This is an RLV command. If you don't have RLV activated, you will see it in local chat.
So either activate RLV in your viewer (in Firestorm, in the viewer menu: Avatar > Preferences > Firestorm > Extras, tick "Allow Remote Scripted Viewer Controls (RLVa)) and restart, or, if you don't want to use Wardrobe for dressing, you don't actually need to wear the HUD except when you want to upload snapshots through the HUD.