If you see this error when trying to install OSSEC+ you will need to do the following steps to correct the issue:
Go into the ossec.conf file:vi /var/ossec/etc/ossec.conf
and replace the <rules></rules> section with the following:
<rules><decoder_dir pattern=".xml$">etc/decoders.d</decoder_dir><rule_dir pattern=".xml$">etc/rules.d</rule_dir><list>etc/lists/threat</list></rules>
Once that has been replaced, run oum update or systemctl restart ossec-hids