From https://soundcloud.com:
SoundCloud is available on Web, iOS, Android, Sonos, Chromecast, and Xbox One.
... no mention of Roku.
Channel apps for Roku are created by the channels themselves and supplied to Roku. If you'd like to have SoundCloud available for Roku, let SoundCloud know.
Roku Community Streaming Expert
I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well.
If this post solves your problem please help others find this answer by clicking "Accept as Solution.".
Ultra 4800 | Streaming Stick 4K+ 3821 | TCL Roku TV 43S245/C107X