That screen shot does not demonstrate an unreachable Splunk UI at port 8000. At best, it says the PATH environment variable is not set to look in the current directory or in /opt/splunk/splunk/bin (an unusual location, BTW) for commands.
With the current directory shown you can use these commands:
./splunk enable boot-start
The first will fail, however, because you are not root. Switch to root or use sudo to enable boot-start.
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