Make sure the Plex app is on the same local network as the Plex Media Server and/or signed in to the same Plex account. Channels are not available for users remotely accessing shared media servers.
Enabling the setting for "Require Authentication on local networks" will prevent channels from showing up on Plex apps even on the same local network unless they are signed in to the same Plex account as the server.
Occasionally, channels will be slow to load when opening a Plex App. Depending on which App is being used, refreshing a couple of times or quitting and restarting the app is necessary for the full list of installed channels to load.