Unfortunately SoundCloud discontinued the API that AppMachine apps used to play music hosted on the platform. Prior to this, SoundCloud already stopped giving out API keys to users for reasons unknown to us.
To implement SoundCloud in your app there's a workaround available in the form of Media RSS feeds. Please read the following article to see whether it's a viable solution for you: https://help.soundcloud.com/hc/en-us/articles/115003451347
Please note that you do need access to the SoundCloud account and the hosted music to be able to set this up.
To add a Media RSS feed to your AppMachine app use the "Media RSS" option after adding a Music block to your app.