It seems you have a missing key for the repository http://deb.debian.org/debian stretch-backports InRelease, it’s not Wazuh related. To add the corresponding key, you could use the command apt-key adv --keyserver keyserver.ubuntu.com --recv 648ACFD622F3D138, which downloads and adds the repository key to the debian keyring.
Hope it helps.
Regards,
Alberto R