tiwaarijee@tiwari-jee-Vostro-15-3568:~$ sudo apt-get update
Get:1 http: //security .ubuntu.com/ubuntu lunar-security InRelease [109 kB]
Get:2 http: //archive. ubuntu.com/ubuntu lunar InRelease [267 kB]
Err:1 http: //security.ubuntu.com/ubuntu lunar-security InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 871920D1991BC93C
Err:2 http: //archive. ubuntu.com/ubuntu lunar InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 871920D1991BC93C
Get:3 http: //archive. ubuntu.com/ubuntu lunar-updates InRelease [109 kB]
Get:4 http: //archive. ubuntu.com/ubuntu lunar-backports InRelease [99.8 kB]
Err:3 http: //archive. ubuntu.com/ubuntu lunar-updates InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 871920D1991BC93C
Err:4 http: //archive. ubuntu.com/ubuntu lunar-backports InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 871920D1991BC93C
Reading package lists... Done
W: GPG error: http://security.ubuntu.com/ubuntu lunar-security InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 871920D1991BC93C
E: The repository 'http://security .ubuntu.com/ubuntu lunar-security 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.
W: GPG error: http://archive .ubuntu.com/ubuntu lunar InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 871920D1991BC93C
E: The repository 'http://archive .ubuntu.com/ubuntu lunar 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.
W: GPG error: http://archive.ubuntu.com/ubuntu lunar-updates InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 871920D1991BC93C
E: The repository 'http://archive.ubuntu.com/ubuntu lunar-updates 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.
W: GPG error: http://archive.ubuntu.com/ubuntu lunar-backports InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 871920D1991BC93C
E: The repository 'http://archive.ubuntu.com/ubuntu lunar-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.
