Can't remove document file from Media
You probably need to delete or remove the file from any nodes, or webform submissions that it references. There should be a usage tab on the actual media resource. I don't have a media site handy so I can't provide screenshots. Once that usage tab is empty, it should be possible to delete that file. Otherwise I'd check the recent log messages in admin/reports to see if there's a file permissions issue, like the only file owners can delete files.