I recently ran into a similar thing with their streaming service, Apple TV Plus. Started a trial and wanted to add a profile for my wife. Browsed around the site for how to do that but couldn't find anything, so I googled and found apparently the only way to do it is to use the "family sharing" app on an Apple device. Apparently while you can subscribe and stream via the web, you can only add family profiles on an Apple device. For some reason.