Music content on the Spotify service can become unavailable often, and there can be many factors for why, music licenses needing updated for some releases, an error with the uploaded content, and / or the label or artist has decided to pull the content from the Spotify service.
Information below about missing music content on Spotify:
Spotify wants all the world's music, and they're working hard on making this happen for you. However sometimes agreements can’t be reached with the artist or label or a change may happen in music ownership.
Spotify adds thousands of albums and artists are added every day. If you can’t find an artist you’re looking for, they may appear (or reappear) soon. In the meantime, feel free to contact the artist or their label to ask when they plan to be on Spotify. https://support.spotify.com/article/Content-missing/