Reused Content is not about Copyright (or permission, or fair use, etc) and is separate from YouTube’s Copyright enforcement–it’s part of the Content Quality section of YPP policies. Here’s more info and best practices to help you make changes: https://yt.be/help/reused-content …
Generally, if it's your own original music then that's fine but if you upload content from multiple sources, or reupload/repurpose someone else’s existing content (ie “reuse content”) https://yt.be/help/reused-content …