Let's add any songs you want to Spotify even though they may not be on Spotify already.
You can add local files to Spotify to add a certain song or MP3 file that you have on your Mac or PC computer or iPhone or Android. In this video, I walk you through the steps to add local files, select the songs and add them to Spotify. Keep in mind, this will only add that song to your specific Spotify, not the global playlist. Best of all, you can always remove that song or play it offline if you want as well.
I hope this video helped solve your problem. Is the app still not working? If so, drop your question in a comment below!