Client Only
Client Only is a mode that does not allow to any player run a server over any hypothesis.
Client Only mode must be used when you have an official and your game build not offer the possibility to run a LAN or personal server.
In Client Only mode, players must have a server to connect always, nonetheless, you can also provide a copy of your server to be hosted by clients, but needs to be a separate build of Authoritative or Relay modes.
To use Client Only mode you must ensure that the "Client Only" option is selected on NetworkManager.