:~$ sudo apt update
Get:1 http://security.debian.org stretch/updates InRelease [53.0 kB]
Ign:2 http://deb.debian.org/debian stretch InRelease
Hit:3 http://downloads-global.3cx.com/downloads/debian stretch-testing InRelease
Get:4 http://deb.debian.org/debian stretch-updates InRelease [93.6 kB]
Hit:5 http://downloads-global.3cx.com/downloads/debian stretch InRelease
Get:6 http://deb.debian.org/debian stretch-backports InRelease [91.8 kB]
Get:7 http://deb.debian.org/debian stretch Release [118 kB]
Get:8 http://deb.debian.org/debian stretch Release.gpg [3,177 B]
Get:9 http://security.debian.org stretch/updates/main Sources [407 kB]
Get:10 http://security.debian.org stretch/updates/main amd64 Packages [771 kB]
Get:11 http://security.debian.org stretch/updates/main Translation-en [362 kB]
Get:12 http://packages.cloud.google.com/apt cloud-sdk-stretch InRelease [6,785 B]
Get:13 http://packages.cloud.google.com/apt google-compute-engine-stretch-stable InRelease [5,525 B]
Err:6 http://deb.debian.org/debian stretch-backports InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 648ACFD622F3D138 NO_PUBKEY 0E98404D386FA1D9
Err:12 http://packages.cloud.google.com/apt cloud-sdk-stretch InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY FEEA9169307EA071 NO_PUBKEY 8B57C5C2836F4BEB
Err:13 http://packages.cloud.google.com/apt google-compute-engine-stretch-stable InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY FEEA9169307EA071 NO_PUBKEY 8B57C5C2836F4BEB
Get:14 http://packages.cloud.google.com/apt google-cloud-packages-archive-keyring-stretch InRelease [5,555 B]
Err:14 http://packages.cloud.google.com/apt google-cloud-packages-archive-keyring-stretch InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY FEEA9169307EA071 NO_PUBKEY 8B57C5C2836F4BEB
Fetched 1,918 kB in 1s (1,640 kB/s)Reading package lists...
Done
Building dependency tree
Reading state information...
Done
105 packages can be upgraded. Run 'apt list --upgradable' to see them.
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used.
GPG error: http://deb.debian.org/debian stretch-backports InRelease:
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 648ACFD622F3D138 NO_PUBKEY 0E98404D386FA1D9W:
An error occurred during the signature verification. The repository is not updated and the previous index files will be used.
GPG error: http://packages.cloud.google.com/apt cloud-sdk-stretch InRelease:
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY FEEA9169307EA071 NO_PUBKEY 8B57C5C2836F4BEBW:
An error occurred during the signature verification. The repository is not updated and the previous index files will be used.
GPG error: http://packages.cloud.google.com/apt google-compute-engine-stretch-stable
InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY FEEA9169307EA071 NO_PUBKEY 8B57C5C2836F4BEBW:
An error occurred during the signature verification. The repository is not updated and the previous index files will be used.
GPG error: http://packages.cloud.google.com/apt google-cloud-packages-archive-keyring-stretch
InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY FEEA9169307EA071 NO_PUBKEY 8B57C5C2836F4BEBW:
Failed to fetch http://deb.debian.org/debian/dists/stretch-backports/InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 648ACFD622F3D138 NO_PUBKEY 0E98404D386FA1D9W:
Failed to fetch http://packages.cloud.google.com/apt/dists/cloud-sdk-stretch/InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY FEEA9169307EA071 NO_PUBKEY 8B57C5C2836F4BEBW:
Failed to fetch http://packages.cloud.google.com/apt/dists/google-compute-engine-stretch-stable/InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY FEEA9169307EA071 NO_PUBKEY 8B57C5C2836F4BEBW:
Failed to fetch http://packages.cloud.google.com/apt/dists/google-cloud-packages-archive-keyring-stretch/InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY FEEA9169307EA071 NO_PUBKEY 8B57C5C2836F4BEBW:
Some index files failed to download. They have been ignored, or old ones used instead.