I tried to run apt-get update again, this returns:
root@panel:~# apt-get update
Ign:1 http://dl.google.com/linux/mod-pagespeed/deb stable InRelease
Hit:2 http://dl.google.com/linux/mod-pagespeed/deb stable Release
Ign:4 http://software.virtualmin.com/gpl/ubuntu virtualmin-precise InRelease
Get:5 http://security.ubuntu.com/ubuntu bionic-security InRelease [88.7 kB]
Hit:6 http://ppa.launchpad.net/ondrej/apache2/ubuntu bionic InRelease
Ign:7 http://software.virtualmin.com/gpl/ubuntu virtualmin-universal InRelease
Get:9 http://software.virtualmin.com/gpl/ubuntu virtualmin-precise Release [7269 B]
Hit:10 http://ppa.launchpad.net/ondrej/php/ubuntu bionic InRelease
Get:8 http://mirrors.ubuntu.com/mirrors.txt Mirrorlist [139 B]
Get:14 http://software.virtualmin.com/gpl/ubuntu virtualmin-universal Release [6553 B]
Hit:12 http://mirror.aktkn.sg/ubuntu bionic-updates InRelease
Get:15 http://software.virtualmin.com/gpl/ubuntu virtualmin-precise Release.gpg [198 B]
Get:16 http://software.virtualmin.com/gpl/ubuntu virtualmin-universal Release.gpg [198 B]
Hit:11 http://archive.ubuntu.com/ubuntu bionic InRelease
Ign:15 http://software.virtualmin.com/gpl/ubuntu virtualmin-precise Release.gpg
Ign:16 http://software.virtualmin.com/gpl/ubuntu virtualmin-universal Release.gpg
Hit:13 http://archive.ubuntu.com/ubuntu bionic-backports InRelease
Hit:17 https://apt.bitninja.io/debian bitninja InRelease
Reading package lists... Done
W: GPG error: http://software.virtualmin.com/gpl/ubuntu virtualmin-precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY E8DD3FA0A0BDBCF9
E: The repository 'http://software.virtualmin.com/gpl/ubuntu virtualmin-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.
W: GPG error: http://software.virtualmin.com/gpl/ubuntu virtualmin-universal Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY E8DD3FA0A0BDBCF9
E: The repository 'http://software.virtualmin.com/gpl/ubuntu virtualmin-universal 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.
There's no need to open a new thread, this sounds like the same issue you were posting about here:
https://www.virtualmin.com/node/66218
Sorry, I misunderstand the reported issue. Feel free to close this thread.