NO PUBKEY

apt update
.../...
Reading package lists... Done
W: GPG error: http://downloads.linux.hpe.com/SDR/repo/mcp focal/current Release: The following signatures couldn't be 
verified because the public key is not available: 
NO_PUBKEY C208ADDE26C2B797
E: The repository 'http://downloads.linux.hpe.com/SDR/repo/mcp focal/current Release' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
Solution:
A) sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys C208ADDE26C2B797
B) deb [trusted=yes] http://www.your_not_trustedcert.org jessie main

sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys  D27D666CD88E42B4
Warning: apt-key is deprecated. Manage keyring files in trusted.gpg.d instead (see apt-key(8)).
Executing: /tmp/apt-key-gpghome.nxEak71UaN/gpg.1.sh --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 
D27D666CD88E42B4
gpg: key D27D666CD88E42B4: public key "Elasticsearch (Elasticsearch Signing Key) <[email protected]>" 
imported
gpg: Total number processed: 1
gpg:               imported: 1

See also

• apt-key adv, NO_PUBKEY