Open the splunkd service and check the Log On tab. Is the service running as the local system or a user?
If your are running as a user, then review this from the Installation Manual:
Important: If you choose to run Splunk as another user, that user must:
•Be a member of an Active Directory domain (you cannot install Splunk as a local machine account other than the Local System account)
•Have local administrator privileges on the machine which you are performing the installation, and
•Have specific user rights, and other additional permissions, depending on the kinds of data you want to collect from remote machines.