I didn't know this but I see the same with 6.2.9 GUI. Apparently you need to use CLI:
xxxx-fg1 # config log ?
custom-field Configure custom log fields.
eventfilter Configure log event filters.
fortianalyzer Configure first FortiAnalyzer device.
fortianalyzer-cloud Configure cloud FortiAnalyzer device.
fortianalyzer2 Configure second FortiAnalyzer device.
fortianalyzer3 Configure third FortiAnalyzer device.
fortiguard Configure log for FortiCloud.
gui-display Configure how log messages are displayed on the GUI.
memory Configure memory log.
null-device Configure logging for statistics collection for when no external logging destination, such as FortiAnalyzer, is present (data is not saved).
setting Configure general log settings.
syslogd Configure first syslog device.
syslogd2 Configure second syslog device.
syslogd3 Configure third syslog device.
syslogd4 Configure fourth syslog device.
threat-weight Configure threat weight settings.
webtrends Configure Web trends.
The Fortinet Security Fabric brings together the concepts of convergence and consolidation to provide comprehensive cybersecurity protection for all users, devices, and applications and across all network edges.