root@tet0-Lenovo-G500s:/home/tet0# apt-get update
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5
Get:10 http://us.archive.ubuntu.com/ubuntu precise-updates InRelease [55.7 kB]
Err:10 http://us.archive.ubuntu.com/ubuntu precise-updates InRelease
W: GPG error: http://archive.canonical.com/ubuntu precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5
E: The repository 'http://us.archive.ubuntu.com/ubuntu precise Release' 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.
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precisein the output. Just posting terminal output isn't enough. – DK Bose Apr 12 '20 at 00:48sudo apt update. That should fix your problem. – user535733 Apr 12 '20 at 01:17root. Unless you're 100% sure of what you're doing, running asrootis inadvisable. The safer route is to usesudowhenever you need elevated privileges. – DK Bose Apr 12 '20 at 04:22