If after the linux apt update command you receive an error similar to what is seen below:

root@epyc:~# apt update
Get:1 http://security.ubuntu.com/ubuntu focal-security InRelease [114 kB]
Ign:2 http://dl.google.com/linux/mod-pagespeed/deb stable InRelease
Hit:3 http://ppa.launchpad.net/ondrej/php/ubuntu focal InRelease
Get:4 http://dl.google.com/linux/mod-pagespeed/deb stable Release [2154 B]
Hit:5 http://de.archive.ubuntu.com/ubuntu focal InRelease
Get:6 http://dl.google.com/linux/mod-pagespeed/deb stable Release.gpg [819 B]
Get:7 http://de.archive.ubuntu.com/ubuntu focal-updates InRelease [114 kB]
Get:8 http://de.archive.ubuntu.com/ubuntu focal-backports InRelease [108 kB]
Ign:6 http://dl.google.com/linux/mod-pagespeed/deb stable Release.gpg
Get:9 https://deb.goaccess.io focal InRelease [2345 B]
Err:9 https://deb.goaccess.io focal InRelease
The following signatures were invalid: EXPKEYSIG 97BD1A0133449C3D Gerardo Orellana <[email protected]>
Reading package lists… Done
W: GPG error: http://dl.google.com/linux/mod-pagespeed/deb stable Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 78BD65473CB3BD13
E: The repository ‘http://dl.google.com/linux/mod-pagespeed/deb stable 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.
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://deb.goaccess.io focal InRelease: The following signatures were invalid: EXPKEYSIG 97BD1A0133449C3D Gerardo Orellana <[email protected]>

Newer solution, UPDATE 2).

I found that it was actually much simpler to let linux solve this problem by itself and for this I used the following commands, because a simple apt update/upgrade no longer worked:

• step 1).

wget -O - https://deb.goaccess.io/gnugpg.key | gpg --dearmor | sudo tee /usr/share/keyrings/goaccess.gpg >/dev/null
echo "deb [signed-by=/usr/share/keyrings/goaccess.gpg arch=$(dpkg --print-architecture)] https://deb.goaccess.io/ $(lsb_release -cs) main" | sudo tee /etc/apt/sources.list.d/goaccess.list
sudo apt-get update

• step 2).

sudo apt upgrade --fix-missing

Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
~~~~~~~~~~~
Need to get 940 kB/218 MB of archives.
After this operation, 213 MB of additional disk space will be used.
Do you want to continue? [Y/n] y
Err:1 https://deb.goaccess.io focal/main amd64 goaccess amd64 1:1.5.7-focal
404 Not Found [IP: 67.205.130.138 443]
Reading changelogs... Done
Extracting templates from packages: 100%
Preconfiguring packages ...
(Reading database ... 154850 files and directories currently installed.)
Preparing to unpack .../motd-news-config_11ubuntu5.6_all.deb ...
Unpacking motd-news-config (11ubuntu5.6) over (11ubuntu5.4) ...
Preparing to unpack .../libc6-dev_2.31-0ubuntu9.9_amd64.deb ...
~~~~~~~~~~~

* The older solution:

Then I recommend you try the following command to create a GoAccess-specific keyring:

wget -O - https://deb.goaccess.io/gnugpg.key | 
  sudo apt-key --keyring /etc/apt/trusted.gpg.d/goaccess.gpg add -

then try apt update again!

solution found via: github.com/allinurl/goaccess/issues/1775