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

apt-key adv, NO_PUBKEY