Please make sure you can access http://127.0.0.1:8080. This should access the Ambari UI. If you can access this URL, please then try http://127.0.0.1:8000. This is to access Hue, the Graphical UI to interact with HDP. If that URL does not work, please ssh into the sandbox and execute service hue restart. That will ensure hue is up and running. Please then try it again. I hope this helps.