The following signatures couldn't be verified because the public key is not available: NO_PUBKEY xxxxxx

just update the APT key:

curl -4 http://download.opensuse.org/repositories/home:/laszlo_budai:/syslog-ng/xUbuntu_16.04/Release.key | sudo apt-key add -

(if you are using other version of Debian/Ubuntu, set the correct version)

and then apt-get update && apt-get upgrade

regards,

Laszlo Budai