Public key is not available - what does that mean?

Hello.

I recently updated my server to Openmediavault 5 (which is based upon Debian "Buster" 10) on my Odroid HC2.

I noticed that the reinstallation of TVHeadend, on ARM devices running Buster, was a bit different than before and ended up on this page for instructions:

https://tvheadend.org/projects/tvhea...ies#Ubuntu-PPA

confirming that there is an unofficial Michale Marley PPA, offering regular updates.

I tried to add the unstable version but am getting the following error:

Code:root@OdroidServer:~# sudo apt-add-repository ppa:mamarley/tvheadend-git
Contains automatic daily builds of the TVHeadend master branch.
More info: https://launchpad.net/~mamarley/+arc.../tvheadend-git
Press [ENTER] to continue or ctrl-c to cancel adding it

gpg: keybox '/tmp/tmpjkbcub4q/pubring.gpg' created
gpg: /tmp/tmpjkbcub4q/trustdb.gpg: trustdb created
gpg: key 26F4EF8440618B66: public key "Launchpad PPA for Michael Marley" imported
gpg: Total number processed: 1
gpg: imported: 1
gpg: no valid OpenPGP data found.
root@OdroidServer:~# sudo apt update
Get:1 file:/var/cache/openmediavault/archives InRelease
Ign:1 file:/var/cache/openmediavault/archives InRelease
Get:2 file:/var/cache/openmediavault/archives Release
Ign:2 file:/var/cache/openmediavault/archives Release
Get:3 file:/var/cache/openmediavault/archives Packages
Ign:3 file:/var/cache/openmediavault/archives Packages
Get:4 file:/var/cache/openmediavault/archives Contents (deb)
Ign:4 file:/var/cache/openmediavault/archives Contents (deb)
Get:5 file:/var/cache/openmediavault/archives Contents (deb)
Ign:5 file:/var/cache/openmediavault/archives Contents (deb)
Get:3 file:/var/cache/openmediavault/archives Packages
Ign:3 file:/var/cache/openmediavault/archives Packages
Get:4 file:/var/cache/openmediavault/archives Contents (deb)
Ign:4 file:/var/cache/openmediavault/archives Contents (deb)
Get:5 file:/var/cache/openmediavault/archives Contents (deb)
Ign:5 file:/var/cache/openmediavault/archives Contents (deb)
Get:3 file:/var/cache/openmediavault/archives Packages
Ign:3 file:/var/cache/openmediavault/archives Packages
Get:4 file:/var/cache/openmediavault/archives Contents (deb)
Ign:4 file:/var/cache/openmediavault/archives Contents (deb)
Get:5 file:/var/cache/openmediavault/archives Contents (deb)
Ign:5 file:/var/cache/openmediavault/archives Contents (deb)
Get:3 file:/var/cache/openmediavault/archives Packages
Ign:3 file:/var/cache/openmediavault/archives Packages
Get:4 file:/var/cache/openmediavault/archives Contents (deb)
Ign:4 file:/var/cache/openmediavault/archives Contents (deb)
Get:5 file:/var/cache/openmediavault/archives Contents (deb)
Ign:5 file:/var/cache/openmediavault/archives Contents (deb)
Get:3 file:/var/cache/openmediavault/archives Packages
Ign:3 file:/var/cache/openmediavault/archives Packages
Get:4 file:/var/cache/openmediavault/archives Contents (deb)
Ign:4 file:/var/cache/openmediavault/archives Contents (deb)
Get:5 file:/var/cache/openmediavault/archives Contents (deb)
Ign:5 file:/var/cache/openmediavault/archives Contents (deb)
Get:3 file:/var/cache/openmediavault/archives Packages
Ign:3 file:/var/cache/openmediavault/archives Packages
Get:4 file:/var/cache/openmediavault/archives Contents (deb)
Ign:4 file:/var/cache/openmediavault/archives Contents (deb)
Get:5 file:/var/cache/openmediavault/archives Contents (deb)
Ign:5 file:/var/cache/openmediavault/archives Contents (deb)
Get:3 file:/var/cache/openmediavault/archives Packages
Hit:6 http://deb.debian.org/debian buster InRelease
Hit:7 http://security.debian.org buster/updates InRelease
Hit:8 http://security.debian.org/debian-security buster/updates InRelease
Get:9 http://ppa.launchpad.net/mamarley/tvheadend-git/ubuntu groovy InRelease [23.8 kB]
Hit:10 http://httpredir.debian.org/debian buster-backports InRelease
Hit:11 http://packages.openmediavault.org/public usul InRelease
Hit:12 http://deb.debian.org/debian buster-updates InRelease
Get:4 file:/var/cache/openmediavault/archives Contents (deb)
Ign:4 file:/var/cache/openmediavault/archives Contents (deb)
Hit:13 https://download.docker.com/linux/debian buster InRelease
Get:5 file:/var/cache/openmediavault/archives Contents (deb)
Ign:5 file:/var/cache/openmediavault/archives Contents (deb)
Ign:14 https://dl.bintray.com/openmediavaul...evelopers/usul buster InRelease
Ign:15 https://dl.bintray.com/openmediavaul...s/usul-testing buster InRelease
Ign:16 https://dl.bintray.com/openmediavaul...rs/usul-extras buster InRelease
Get:17 https://dl.bintray.com/openmediavaul...evelopers/usul buster Release [4363 B]
Get:18 https://dl.bintray.com/openmediavaul...s/usul-testing buster Release [3538 B]
Get:19 https://dl.bintray.com/openmediavaul...rs/usul-extras buster Release [3538 B]
Hit:20 https://openmediavault.github.io/packages usul InRelease
Hit:21 https://mirrors.dotsrc.org/armbian-apt buster InRelease
Err:9 http://ppa.launchpad.net/mamarley/tvheadend-git/ubuntu groovy InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 26F4EF8440618B66
Reading package lists... Done
W: GPG error: http://ppa.launchpad.net/mamarley/tvheadend-git/ubuntu groovy InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 26F4EF8440618B66
E: The repository 'http://ppa.launchpad.net/mamarley/tvheadend-git/ubuntu groovy 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.I would be really grateful for some advice on what to do - it is safe to install this repostiory and, if so, how can I get around the key issue?

Thanks very much.