Good question. There's YouTube, which has a music section, but I'm not sure if this is the same as YouTube music. It has my playlists from youtube music, but if I search for an artist or album I only get videos.
YouTube Music should have its own Roku channel, just like Pandora and Spotify. I want to be able to access YouTube Music on my TV in the same manner I access it on my laptop and phone. After all, I am paying $15 a month for it. This makes no sense.