Hello @Hoshcof and welcome to the community
Unfortunately, less than a year ago, Prince's publisher asked to Spotify to remove all his catalog from the streaming service.
Availability of music on Spotify is up to the artists (or representatives) and / or their labels. Music missing can be a decision of one of them.
Spotify wants all the music in the world to stream and works hard daily to try to convince artists and labels to let all their catalog in streaming. Unfortunately, sometimes an agreement can't be found.
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Note: I'm not a Spotify employee.