Hello World

$ sudo apt-get update
Get:1 http://dl.google.com/linux/chrome/deb stable InRelease [1811 B]
Err:1 http://dl.google.com/linux/chrome/deb stable InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 4EB27DB2A3B88B8B

Get:2 http://security.ubuntu.com/ubuntu focal-security InRelease [114 kB]
Get:3 http://mirrors.ubuntu.com/mirrors.txt Mirrorlist [425 B]
Hit:4 http://ftp.tku.edu.tw/ubuntu focal InRelease
Get:6 http://ftp.tku.edu.tw/ubuntu focal-backports InRelease [108 kB]
Get:5 http://ubuntu.cs.nctu.edu.tw/ubuntu focal-updates InRelease [114 kB]
Get:7 http://mirror01.idc.hinet.net/ubuntu focal-updates/universe amd64 Packages [1314 kB]
Ign:10 http://mirror.nwlab.tk/ubuntu focal-updates/multiverse amd64 Packages
Get:10 http://ftp.tku.edu.tw/ubuntu focal-updates/multiverse amd64 Packages [31.2 kB]
Ign:8 http://mirror01.idc.hinet.net/ubuntu focal-updates/main amd64 Packages
Get:9 http://mirror01.idc.hinet.net/ubuntu focal-updates/restricted amd64 Packages [2141 kB]
Ign:8 https://mirror.ossplanet.net/ubuntu focal-updates/main amd64 Packages
Ign:8 http://mirror.nwlab.tk/ubuntu focal-updates/main amd64 Packages
Ign:8 http://ftp.tku.edu.tw/ubuntu focal-updates/main amd64 Packages
Ign:8 http://ftp.ubuntu-tw.net/ubuntu focal-updates/main amd64 Packages
Ign:8 https://tw1.mirror.blendbyte.net/ubuntu focal-updates/main amd64 Packages
Ign:8 https://ubuntu.ccns.ncku.edu.tw/ubuntu focal-updates/main amd64 Packages
Get:8 http://archive.ubuntu.com/ubuntu focal-updates/main amd64 Packages [3026 kB]
Reading package lists... Done
W: GPG error: http://dl.google.com/linux/chrome/deb stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 4EB27DB2A3B88B8B
E: The repository 'http://dl.google.com/linux/chrome/deb stable 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.

解決方式
$ sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 4EB27DB2A3B88B8B

Executing: /tmp/apt-key-gpghome.bXgfotSCrB/gpg.1.sh --keyserver keyserver.ubuntu.com --recv-keys 4EB27DB2A3B88B8B

gpg: key 7721F63BD38B4796: "Google Inc. (Linux Packages Signing Authority) <linux-packages-keymaster@google.com>" 2 new signatures
gpg: key 7721F63BD38B4796: "Google Inc. (Linux Packages Signing Authority) <linux-packages-keymaster@google.com>" 2 new subkeys
gpg: Total number processed: 1
gpg:            new subkeys: 2
gpg:         new signatures: 2

重新執行一次
$ sudo apt-get update
Get:1 http://dl.google.com/linux/chrome/deb stable InRelease [1811 B]
Get:2 http://dl.google.com/linux/chrome/deb stable/main amd64 Packages [1080 B]
Get:3 http://security.ubuntu.com/ubuntu focal-security InRelease [114 kB]
Get:4 http://mirrors.ubuntu.com/mirrors.txt Mirrorlist [425 B]
Hit:5 http://ubuntu.cs.nctu.edu.tw/ubuntu focal InRelease
Hit:6 http://ftp.tku.edu.tw/ubuntu focal-updates InRelease
Hit:7 http://ftp.mirror.tw/pub/ubuntu/ubuntu focal-backports InRelease
Fetched 115 kB in 2s (69.9 kB/s)
Reading package lists... Done