Seems as though the bootstart user cannot run the service? Here is a link for running splunk as a systemd service: https://docs.splunk.com/Documentation/Splunk/8.2.4/Admin/RunSplunkassystemdservice
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May need to configure the "boot-start" user. Here is the link: https://docs.splunk.com/Documentation/Splunk/8.2.4/Admin/ConfigureSplunktostartatboottime#Enable_boot-start_as_a_non-root_user
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