How do I fix the GPG error "NO_PUBKEY"?I added some extra repositories with the Software Sources program. But when I reload the package database, I get an error like the following: W: GPG error: http://ppa.launchpad.net trusty InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8BAF9A6F I know I can fix it using apt-key in a terminal, according to the official Ubuntu documentation
How to install 'add-apt-repository' using the terminal?
Trouble downloading packages list due to a "Hash sum mismatch" error
'Some index files failed to download' while updating
