Check if the license.txt file exists on the system
The default for the license.txt file is the /opt/syslog-ng/etc/ directory.
You can also run the following on Linux systems:
$ /opt/syslog-ng/sbin/syslog-ng-ctl show-license-info
If there is no license found, install one.
Syslog-ng will need to be restarted to find the license.