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. But I would have liked to do it graphically. Is there a mean not to open a terminal?