The Debian Buster Foreman repositories seem to be throwing an error related to the signing key.
Err:9 http://deb.theforeman.org buster InRelease
The following signatures were invalid: EXPKEYSIG 6F8600B9563278F6 Foreman Automatic Signing Key (2016) packages@theforeman.org
Err:10 http://deb.theforeman.org plugins InRelease
The following signatures were invalid: EXPKEYSIG 6F8600B9563278F6 Foreman Automatic Signing Key (2016) packages@theforeman.org
Reading package lists… Done
W: GPG error: http://deb.theforeman.org buster InRelease: The following signatures were invalid: EXPKEYSIG 6F8600B9563278F6 Foreman Automatic Signing Key (2016) packages@theforeman.org
E: The repository ‘http://deb.theforeman.org buster InRelease’ is no longer 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://deb.theforeman.org plugins InRelease: The following signatures were invalid: EXPKEYSIG 6F8600B9563278F6 Foreman Automatic Signing Key (2016) packages@theforeman.org
E: The repository ‘http://deb.theforeman.org plugins InRelease’ is no longer signed.
N: Updating from such a repository can’t be done securely, and is therefore disabled by default.