We can’t delete a release, because we need its details to pay royalties. If a release is no longer in stores, you can hide it instead.
In the menu on the left, select Library, then the release.
Check that its status is Cancelled, Removing From Stores, or Removed from Stores. Still live? Remove it from stores first.
Select Release actions ▾, then Hide release, and confirm in the window that opens.
The release no longer shows in your Library. You still get all royalties owed for it.