我下载了最新的ubuntu-20.04-live-server-amd64.iso并开始安装。当我到达存储配置部分时，我选择了“自定义布局”。磁盘已经有分区，所以我尝试删除不需要的分区。选择后Delete会弹出此消息：Cannot delete a single partition from a device that already has partitions。
问题是我该如何删除它们？
server partitioning system-installation usb-installation 20.04