Best answer by Mathis
Thank you for posting this on our help forum. I've reached out to our engineers about this. They've confirmed that the error message indicates that the deletion has either failed or is in process. I.e., there might currently be a bit of a delay in the deletion process.
If you try and delete the tracks again in a few hours (that is, if they are still around by that time), they should be deleted automatically.
Sorry about the troubles, hope this'll do the trick.