sudo apt-get update GPG Error
apt-get update
Get:1 https://developer.download.nvidia.com/compute/cuda/repos/ubuntu2004/x86_64 InRelease [1581 B]
Err:1 https://developer.download.nvidia.com/compute/cuda/repos/ubuntu2004/x86_64 InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A4B469963BF863CC
Ign:2 https://developer.download.nvidia.com/compute/machine-learning/repos/ubuntu2004/x86_64 InRelease
Get:3 http://security.ubuntu.com/ubuntu focal-security InRelease [128 kB]
Hit:4 http://archive.ubuntu.com/ubuntu focal InRelease
Ign:5 https://developer.download.nvidia.com/compute/machine-learning/repos/ubuntu1804/x86_64 InRelease
Hit:6 https://developer.download.nvidia.com/compute/machine-learning/repos/ubuntu2004/x86_64 Release
Hit:7 https://developer.download.nvidia.com/compute/machine-learning/repos/ubuntu1804/x86_64 Release
Get:9 http://archive.ubuntu.com/ubuntu focal-updates InRelease [128 kB]
Get:11 http://security.ubuntu.com/ubuntu focal-security/main amd64 Packages [3746 kB]
Get:12 http://archive.ubuntu.com/ubuntu focal-backports InRelease [128 kB]
Get:13 http://archive.ubuntu.com/ubuntu focal-updates/multiverse amd64 Packages [33.5 kB]
Get:14 http://archive.ubuntu.com/ubuntu focal-updates/universe amd64 Packages [1512 kB]
Get:15 http://archive.ubuntu.com/ubuntu focal-updates/main amd64 Packages [4218 kB]
Get:16 http://security.ubuntu.com/ubuntu focal-security/universe amd64 Packages [1227 kB]
Get:17 http://security.ubuntu.com/ubuntu focal-security/restricted amd64 Packages [3712 kB]
Get:18 http://security.ubuntu.com/ubuntu focal-security/multiverse amd64 Packages [30.9 kB]
Get:19 http://archive.ubuntu.com/ubuntu focal-updates/restricted amd64 Packages [3867 kB]
Get:20 http://archive.ubuntu.com/ubuntu focal-backports/main amd64 Packages [55.2 kB]
Get:21 http://archive.ubuntu.com/ubuntu focal-backports/universe amd64 Packages [28.6 kB]
Reading package lists... Done
W: GPG error: https://developer.download.nvidia.com/compute/cuda/repos/ubuntu2004/x86_64 InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A4B469963BF863CC
E: The repository 'https://developer.download.nvidia.com/compute/cuda/repos/ubuntu2004/x86_64 InRelease' is no longer signed.
solution
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys A4B469963BF863CC
참고)