Siri requires Apple Music subscription to answer music-related questions
Many Apple Music subscribers, one of which is Pandora's co-creator Tom Conrad, have noticed that Siri isn't too keen on answering some questions unless a subscription to Apple Music is present. For example, searching for most popular songs in any location or a time period isn't an option for non Apple Music users.
Instead of displaying an alternative web search result, Siri right out says that she's not able to perform the look up.
Sorry, I can't look up the music charts for you. You don't seem to be subscribed to Apple Music.