chkconfig is (breifly explained) just a functionality that creates symlinks to the initscript in the different startup runlevel folders, and also provides an overview of what services that are started at different runlevels.
"runlevels" no longer exist for systems using systemd (or related init systems), so perhaps SuSE removed that option?
There is on the other hand no need to run the splunk script to make splunk start after boot....just use your systems built in auto start mechanisms and "point it" at the splunk binary $SPLUNK_HOME/bin/splunk
(the splunk enable boot-start functionality just creates a init script and, sets the runlevels for which splunk should run in that script and then use chkconfig with the same information to make the system changes at once.
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