I'm guessing your forwarder runs as root, or some other user. Check with ps -Af | grep splunkd or similar to confirm.
Assuming you don't need the forwarder to run as root (if you do, work to remove that need), you should stop the forwarder, chown all files to splunk, run splunk enable boot-start -user splunk, and start splunk from the user splunk.
Thanks Martin for your response. Thought I already did that in the way you told. However, the issue was, there was no home directory for splunk user so there was home directory. After I created the home directory for splunk and ran the command the issue got resolved.