iro:~$ sudo add-apt-repository universe
[sudo] password for aidin:
Adding component(s) 'universe' to all repositories.
Press [ENTER] to continue or Ctrl-c to cancel.
Get:1 http://mirror.rasanegar.com/ubuntu/archive xenial InRelease [247 kB]
Hit:6 https://packages.microsoft.com/ubuntu/21.04/prod hirsute InRelease
Get:2 http://mirror.rasanegar.com/ubuntu/archive xenial-security InRelease [99.8 kB]
Get:3 http://mirror.rasanegar.com/ubuntu/archive xenial-updates InRelease [99.8 kB]
Err:1 http://mirror.rasanegar.com/ubuntu/archive xenial InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32
Get:4 http://mirror.rasanegar.com/ubuntu/archive xenial-proposed InRelease [251 kB]
Err:2 http://mirror.rasanegar.com/ubuntu/archive xenial-security InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32
Get:5 http://mirror.rasanegar.com/ubuntu/archive xenial-backports InRelease [97.4 kB]
Err:3 http://mirror.rasanegar.com/ubuntu/archive xenial-updates InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32
Err:4 http://mirror.rasanegar.com/ubuntu/archive xenial-proposed InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32
Err:5 http://mirror.rasanegar.com/ubuntu/archive xenial-backports InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32
Reading package lists... Done
W: GPG error: http://mirror.rasanegar.com/ubuntu/archive xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32
E: The repository 'http://mirror.rasanegar.com/ubuntu/archive xenial 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://mirror.rasanegar.com/ubuntu/archive xenial-security InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32
E: The repository 'http://mirror.rasanegar.com/ubuntu/archive xenial-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://mirror.rasanegar.com/ubuntu/archive xenial-updates InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32
E: The repository 'http://mirror.rasanegar.com/ubuntu/archive xenial-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://mirror.rasanegar.com/ubuntu/archive xenial-proposed InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32
E: The repository 'http://mirror.rasanegar.com/ubuntu/archive xenial-proposed 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://mirror.rasanegar.com/ubuntu/archive xenial-backports InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32
E: The repository 'http://mirror.rasanegar.com/ubuntu/archive xenial-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.