The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY
just run :
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys yourpublickey
apt-get update
reference link : https://chrisjean.com/fix-apt-get-update-the-following-signatures-couldnt-be-verified-because-the-public-key-is-not-available/