gnupg - GPG error: The following signatures couldn't be verified because the public key is not available

after sudo apt-get update I get this error:

W: GPG error: http://archive.hivemind.me all Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3C962022012520A0

Run the following in your terminal,

 sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 3C962022012520A0
 sudo apt-get update

References: How do I fix the GPG error "NO_PUBKEY"?

You need to replace the key (3C962022...) with the one that is displayed in the error message in the terminal.