after install some PPAs when ever I do sudo apt-get update
the output is
Hit:1 http://au.archive.ubuntu.com/ubuntu bionic InRelease
Hit:2 http://repo.steampowered.com/steam precise InRelease
Hit:3 http://au.archive.ubuntu.com/ubuntu bionic-updates InRelease
Hit:4 http://au.archive.ubuntu.com/ubuntu bionic-backports InRelease
Hit:5 http://security.ubuntu.com/ubuntu bionic-security InRelease
Ign:6 http://ppa.launchpad.net/grumbel/ppa/ubuntu bionic InRelease
Hit:7 http://ppa.launchpad.net/kelleyk/emacs/ubuntu bionic InRelease
Err:8 http://ppa.launchpad.net/grumbel/ppa/ubuntu bionic Release
404 Not Found [IP: 91.189.95.83 80]
Get:9 https://qgis.org/ubuntu bionic InRelease [3,693 B]
Err:9 https://qgis.org/ubuntu bionic InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 51F523511C7028C3
Reading package lists... Done
E: The repository 'http://ppa.launchpad.net/grumbel/ppa/ubuntu bionic Release' does not have a Release file.
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: https://qgis.org/ubuntu bionic InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 51F523511C7028C3
E: The repository 'https://qgis.org/ubuntu bionic 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.