After installing Splunk, why am I receiving the following error: “failed to start splunk.service: unit splunk.service not found”

if you are sure that you’ve installed splunk in opt directory
then go to bin directory like this: cd /opt/splunk/bin/

now run this command from your bin directory: ./splunk start or ./splunk restart

root@aryan-VirtualBox:~# cd /opt/splunk/bin/
root@aryan-VirtualBox:/opt/splunk/bin# ls

root@aryan-VirtualBox:/opt/splunk/bin# ./splunk start
The splunk daemon (splunkd) is already running.
If you get stuck, we’re here to help.
Look for answers here: http://docs.splunk.com

The Splunk web interface is at http://aryan-VirtualBox:8000