Spotify would love to have all the world’s music available to everyone. However, some artists and tracks aren't currently available. I don't know the exact reason wjy that song is missing.
I'd recommend reading this article for more info.
However, you can download that song ad listen to it as a local file in Spotify. You can learn more about local files in Spotify here.
Hope this helps! And make sure to ask any other questions, I'll be happy to help.
Happy listening, Jose 🙂
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Note: I'm not a Spotify employee.