Hi and welcome to the Community @raazanha!
If you want to have a specific artist or song on Spotify, you have to find out which Label this artist has.
Because most of the time the label of a artist decides weather to put or to put not albums or songs on Streaming Services (e.g. Spotify).
You can ask them why they are not on Spotify...
So if a song is not on Spotify it is because of legal reasons. I hope I could help a bit.
Keep It Up!
Welcome to the community! Please read this, it should give more information about why some music is not available:https://community.spotify.com/t5/Content-Questions/I-want-an-artist-song-on-Spotify/td-p/1136005The availability of music on Spotify is up to the artist and their music label.