apt-key adv --keyserver hkp://pool.sks-keyservers.net:80 --recv-keys MISSING_KEY
Error usually is like follow:
GPG error: http://ppa.launchpad.net/nilarimogard/webupd8/ubuntu focal InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 531EE72F4C9D234C
E: The repository 'http://ppa.launchpad.net/nilarimogard/webupd8/ubuntu focal InRelease' is not signed.
E: Failed to download some files
W: Failed to fetch http://ppa.launchpad.net/nilarimogard/webupd8/ubuntu/dists/focal/InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 531EE72F4C9D234C
E: Some index files failed to download. They have been ignored, or old ones used instead.
