Web Serial isn't available in this browser. This device can join a session as a remote; the Arduino must be plugged into a desktop running Chrome or Edge.
Session
Enter the code of an existing session, or generate a new one. The same code works for the host, the phone remote and the showcase.
Host is offline. Nothing connected to the Arduino is reporting on this session.
Another page is already hosting this code (probably the showcase on the Arduino PC). Two hosts will fight over the session. Switch this page to a remote, or use "Change" and generate a new code.