Hey there everyone! The availability of music on Spotify is mainly up to the Record Label or the artist.
Unfortunately, the Record Label or The Artist probably decided to not have their music available for streaming on Spotify or in your region (reasons may include but not limited to: copyright laws, marketing reasons). You should try contacting the Record Label, and ask them why that Album/Song isn't on Spotify
If you want more info on this, you can learn morehere Have a nice day! 🙂