Try the following:
Unable to ping, returns java.lang.UnsupportedOperationException
The workaround for this appears to be:
systemctl edit --full opennms.service
In [Service] section and under Environment="" line, Add the line: AmbientCapabilities=CAP_NET_RAW CAP_NET_BIND_SERVICE
Save this and exit, run systemctl daemon-reload, then restart the opennms service
This seems to be caused by OpenNMS no longer running as root. This line should be added to the systemctl service file so it is present for new installations.