Thanks!

On the one hand, AIMP don't have built-in music library, so you cannot find the streamed content – Spotify music; on the other hand, Spotify use a set of copyright protection system to prevent you from playing their content out of Spotify. Even though the format of Spotify music conforms to the supported formats of AIMP, but all Spotify music have digital rights management protection. Fortunately, I found Tunelf Spotify Music Converter to help me to play Spotify music on AIMP