The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 78BD65473CB3BD13 code example

Example 1: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 6494C6D6997C215E

Example 2: he following signatures couldn't be verified because the public key is not available: NO_PUBKEY 363A96A5CEA9EA27

sudo apt-key adv --keyserver ha.pool.sks-keyservers.net --recv-keys 363A96A5CEA9EA27

Example 3: ubuntu 18.04 jenkins The following signatures couldn't be verified because the public key is not available:

Utils:

https://www.digitalocean.com/community/tutorials/how-to-install-jenkins-on-ubuntu-18-04

https://pkg.jenkins.io/debian-stable/

Solve the problem by adding this:
wget -q -O - https://pkg.jenkins.io/debian-stable/jenkins.io.key | sudo apt-key add -