It might not be admin/changeme. While you were installing splunk, there should be a prompt for providing admin user name and password. You have to use that.
If you don't remember that, you can reset the password. Refer one of the answers - https://answers.splunk.com/answers/834/how-to-reset-the-admin-password.html
--- What goes around comes around. If it helps, hit it with Karma🙂
