I don't know why I am getting this from some days in the top panel, though it is different every time
got this on apt update
Err:16 http://ppa.launchpad.net/ubuntu-mozilla-daily/ppa/ubuntu focal InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY EF4186FE247510BE Reading package lists... Done W: GPG error: http://ppa.launchpad.net/ubuntu-mozilla-daily/ppa/ubuntu focal InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY EF4186FE247510BE E: The repository 'http://ppa.launchpad.net/ubuntu-mozilla-daily/ppa/ubuntu focal 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.
these two methods didn't solve it y-ppa-manager, keyserver