How to 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. 4ABE1AC7557BEFF9.
Then update

sudo apt-get update

ALTERNATE METHOD:

sudo gpg --keyserver pgpkeys.mit.edu --recv-key <PUBKEY>
 sudo gpg -a --export <PUBKEY> | sudo apt-key add -
 sudo apt-get update

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