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I'm trying to install certbot on my Debian 9 server on Google compute engine VM.
Here's what I have done so far:
backports repository, I have edit the
sudo nano /etc/apt/sources.list
Then add this line at the very bottom of the file:
deb http://ftp.debian.org/debian stretch-backports main
After that when I run this command:
sudo apt update
it return this error:
Reading package lists... Done
W: GPG error: http://ftp.debian.org/debian stretch-backports
InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8B48AD6246925553 NO_PUBKEY 7638D0442B90D010
E: The repository 'http://ftp.debian.org/debian stretch-backports 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.