ISRC (International Standard Recording Code) is a unique code that is assigned to a sound recording and is required to enable a track for monetization. Use the same ISRC for a given sound recording wherever you make it available, across all territories and music services. If you don’t already have ISRCs for your tracks, you can have them created for you when uploading or editing a track.
As ISRCs are unique per track, first make sure you don’t already have a code through one of your partners. Ask about ISRCs for your tracks if you work with record labels / distributors / music services but have retained the right to upload, make available, and monetize the tracks on SoundCloud yourself.