The recent 3.0.0-550bionic package for ossec-hids-agent is broken. This is what happens when trying to perform and install from the atomic repo.
Selecting previously unselected package ossec-hids-agent.
(Reading database ... 135330 files and directories currently installed.)
Preparing to unpack .../ossec-hids-agent_3.0.0-5505bionic_amd64.deb ...
Unpacking ossec-hids-agent (3.0.0-5505bionic) ...
Setting up ossec-hids-agent (3.0.0-5505bionic) ...
Adding group `ossec' (GID 120) ...
chown: cannot access '/var/ossec//etc/ossec.conf': No such file or directory
dpkg: error processing package ossec-hids-agent (--configure):
installed ossec-hids-agent package post-installation script subprocess returned error exit status 1
Errors were encountered while processing:
E: Sub-process /usr/bin/dpkg returned an error code (1)
Support/Development for OSSEC