Can't add a Public Key to Ubuntu 22.04

I'm getting an error while trying to update ubuntu. I tried to manually add the key mentioned in the errors to my system by using:

sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 871920D1991BC93C

But it doesn't make any changes and I still get the same error. And I am a bit concerned about my repositories as well. I kinda think I'm missing something here.

    sudo apt-get update
Hit:1 http://archive.canonical.com/ubuntu jammy InRelease
Get:2 http://archive.ubuntu.com/ubuntu jammy InRelease [270 kB]                                         
Hit:3 https://dl.winehq.org/wine-builds/ubuntu jammy InRelease                                          
Hit:4 https://repositories.intel.com/graphics/ubuntu focal InRelease                                    
Get:5 http://archive.ubuntu.com/ubuntu jammy-updates InRelease [119 kB]                  
Get:6 http://archive.ubuntu.com/ubuntu jammy-security InRelease [110 kB]
Get:7 http://archive.ubuntu.com/ubuntu jammy-backports InRelease [107 kB]
Err:1 http://archive.canonical.com/ubuntu jammy InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 871920D1991BC93C
Err:4 https://repositories.intel.com/graphics/ubuntu focal InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9B10C065DBB72B06
Err:2 http://archive.ubuntu.com/ubuntu jammy InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 871920D1991BC93C
Hit:8 https://repositories.intel.com/graphics/ubuntu jammy InRelease
Err:5 http://archive.ubuntu.com/ubuntu jammy-updates InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 871920D1991BC93C
Err:6 http://archive.ubuntu.com/ubuntu jammy-security InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 871920D1991BC93C
Err:7 http://archive.ubuntu.com/ubuntu jammy-backports InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 871920D1991BC93C
Reading package lists... Done
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://archive.canonical.com/ubuntu jammy InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 871920D1991BC93C
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://repositories.intel.com/graphics/ubuntu focal InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9B10C065DBB72B06
W: GPG error: http://archive.ubuntu.com/ubuntu jammy 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 jammy 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 jammy-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 jammy-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 jammy-security 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 jammy-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 jammy-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 jammy-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.

Here is my sources.list output which I think it should not be correct because I changed it myself. I was in a lot trouble with the default one for 22.04, and so I decided to back in time a bit :

# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://archive.ubuntu.com/ubuntu/ focal main restricted
deb-src http://archive.ubuntu.com/ubuntu/ focal main restricted

## Major bug fix updates produced after the final release of the
## distribution.
deb http://archive.ubuntu.com/ubuntu/ focal-updates main restricted
deb-src http://archive.ubuntu.com/ubuntu/ focal-updates main restricted

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://archive.ubuntu.com/ubuntu/ focal universe
deb-src http://archive.ubuntu.com/ubuntu/ focal universe
deb http://archive.ubuntu.com/ubuntu/ focal-updates universe
deb-src http://archive.ubuntu.com/ubuntu/ focal-updates universe

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu 
## team, and may not be under a free licence. Please satisfy yourself as to 
## your rights to use the software. Also, please note that software in 
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://archive.ubuntu.com/ubuntu/ focal multiverse
deb-src http://archive.ubuntu.com/ubuntu/ focal multiverse
deb http://archive.ubuntu.com/ubuntu/ focal-updates multiverse
deb-src http://archive.ubuntu.com/ubuntu/ focal-updates multiverse

## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://archive.ubuntu.com/ubuntu/ focal-backports main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ focal-backports main restricted universe multiverse

## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
# deb http://archive.canonical.com/ubuntu focal partner
# deb-src http://archive

And here is my output for sudo apt-get update after all the changes:

sudo apt-get update
Hit:1 http://archive.ubuntu.com/ubuntu focal InRelease
Hit:2 http://archive.ubuntu.com/ubuntu focal-updates InRelease
Hit:3 http://archive.ubuntu.com/ubuntu focal-backports InRelease
Reading package lists... Done
W: http://archive.ubuntu.com/ubuntu/dists/focal/InRelease: Key is stored in legacy trusted.gpg keyring (/etc/apt/trusted.gpg), see the DEPRECATION section in apt-key(8) for details.
W: http://archive.ubuntu.com/ubuntu/dists/focal-updates/InRelease: Key is stored in legacy trusted.gpg keyring (/etc/apt/trusted.gpg), see the DEPRECATION section in apt-key(8) for details.
W: http://archive.ubuntu.com/ubuntu/dists/focal-backports/InRelease: Key is stored in legacy trusted.gpg keyring (/etc/apt/trusted.gpg), see the DEPRECATION section in apt-key(8) for details.

I still have the problem

In Ubuntu 22.04, a lot has changed with how apt handles keys now. Namely, apt-key is deprecated in favor of direct keyring management for each repository.

I'm going to assume that your main /etc/apt/sources.list file is specifically pointing everything at ubuntu.lafibre.info which is NOT a standard mirror.

The only real way to do this proper nowadays is to download the main keyring keys into either /etc/apt/keyrings/ and then add them one by one to your repositories' settings, or add them to /etc/apt/trusted.gpg.d then anything signed with that key will be trusted:

gpg --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 871920D1991BC93C
gpg --export 871920D1991BC93C | sudo tee /etc/apt/trusted.gpg.d/ubuntu.lafibre.info.gpg

This will first download the key to your standard gnupg keyring. We then take that key and export it in readable format to the file at /etc/apt/trusted.gpg.d/ubuntu.lafibre.info.gpg which should then work when you run sudo apt update.

Run sudo apt update and it should work now.

NOTICE: If you do this, you are trusting that PGP / GPG key to sign ANY repository, so this is not the most secure method. The more secure method is to download this key to /etc/apt/keyrings/ubuntu.lafibre.info.gpg then change every line that uses this URL to say deb [ signed-by=/etc/apt/keyrings/ubuntu.lafibre.info.gpg ] at the beginning instead of deb, which is the more secure approach.