解决方法

首先获取缺少的公钥签名（NO_PUBKEY）。
例子：

The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9578539176BAFBC6
Reading package lists... Done
W: GPG error: http://ppa.launchpad.net/peek-developers/daily/ubuntu eoan InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9578539176BAFBC6

使用apt-key命令通过刚才获取的签名将密钥导入Ubuntu 20.04系统。

例子：

$ sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 9578539176BAFBC6
Executing: /tmp/apt-key-gpghome.fnm4L3G3iP/gpg.1.sh --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 9578539176BAFBC6
gpg: key 9578539176BAFBC6: public key "Launchpad PPA for Peek Developers" imported
gpg: Total number processed: 1
gpg:               imported: 1

执行“APT-Key List”命令列出所有当前导入的密钥，确认已导入公钥：

$ apt-key list
/etc/apt/trusted.gpg
-------------------
pub   rsa4096 2015-02-14 [SC]
      8C95 3129 9E7D F2DC F681  B499 9578 5391 76BA FBC6
uid           [ unknown] Launchpad PPA for Peek Developers
/etc/apt/trusted.gpg.d/ubuntu-keyring-2012-archive.gpg
-----------------------------------------------------
pub   rsa4096 2012-05-11 [SC]
      790B C727 7767 219C 42C8  6F93 3B4F E6AC C0B2 1F32
uid           [ unknown] Ubuntu Archive Automatic Signing Key (2012) 
/etc/apt/trusted.gpg.d/ubuntu-keyring-2012-cdimage.gpg
-----------------------------------------------------
pub   rsa4096 2012-05-11 [SC]
      8439 38DF 228D 22F7 B374  2BC0 D94A A3F0 EFE2 1092
uid           [ unknown] Ubuntu CD Image Automatic Signing Key (2012) 
/etc/apt/trusted.gpg.d/ubuntu-keyring-2015-archive.gpg
-----------------------------------------------------
pub   rsa4096 2015-09-17 [SC]
      F6EC B376 2474 EDA9 D21B  7022 8719 20D1 991B C93C
uid           [ unknown] Ubuntu Archive Automatic Signing Key (2015)

问题

Ubuntu 20.04 GPG error: The following signatures couldn't be verified
“ubuntu 20.04 gpg错误：无法验证以下签名”

这个错误常见于 将第三方包存储库添加到APT包管理器时。