Try to enable it manually by opening music, then press the three dot menu on right top, then select settings, from settings select manage tabs, check whether Spotify is available in that menu. If available tick Spotify to enable it.
I have tried enabling Spotify in samsung music app. It just shows me playlist and once I click any of the playlist from Spotify tab it takes me directly to playstore to install Spotify app. That means Spotify app must be present in the device for playing Spotify music in Samsung's music app. Try installing Spotify first.
