How do I fix the GPG error “NO_PUBKEY”?

Execute the following commands in terminal:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys <PUBKEY>

where <PUBKEY> is your missing public key for repository, e.g. 8BAF9A6F.

Then update in terminal:

sudo apt-get update

H/T Curt J. Sampson