the public key is not available:

截屏2020-10-08 下午2.33.58.png

错误：

Get:1 https://mirrors.ustc.edu.cn/kali kali-rolling InRelease [30.5 kB]
Err:1 https://mirrors.ustc.edu.cn/kali kali-rolling InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY ED444FF07D8D0BF6
Reading package lists... Done
W: GPG error: https://mirrors.ustc.edu.cn/kali kali-rolling InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY ED444FF07D8D0BF6
E: The repository 'https://mirrors.ustc.edu.cn/kali kali-rolling InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

修复：

wget -q -O - https://archive.kali.org/archive-key.asc | apt-key add