Try subscribing to ‘application/#’ in case you got the application id wrong (it should be in the URL of the page you were viewing before).
Or for that matter just subscribe to ‘#’ and see everything including the raw encrypted packets to/from gateways and various housekeeping. Unless you have a huge number of devices nearby it won’t be an extreme flood and will give you some sense of what is / isn’t working.
Perhaps you connected to the wrong broker? You aren’t specifying to presumably it assumes localhost and a default port.
And don’t run mosquitto_sub as root. There’s no reason to - at most the broker would need privileged port access. The client does not; any permission issues are between the client and broker and related plug-ins, not between the client and the host os.