apt error:
NO_PUBKEY 78BD65473CB3BD13
solution:
wget -q -O - https://dl.google.com/linux/linux_signing_key.pub | sudo apt-key add -;sudo apt update

apt error:
"NO_PUBKEY 03C3AD3A7F068E5D"
solution:
curl -fsSL https://mega.nz/keys/MEGA_signing.key | sudo apt-key add -;sudo apt update