Are you SURE that you successfully connected to your database?
One way to do it is: at the OEM Login window choose "connect directly to a database". Enter your username, password and correct service name. If you miss any of these, your connection will fail.
Another way is - when you already are in the SQL*Plus worksheet (which will happen even though your first attempt failed), issue this command: "CONNECT username/password@service_name" and click the "Execute" button on a toolbar. You'll receive a "Connected" message if everything is OK.
Just wondering ... are you sure you installed scott's schema? Can you connect as "sys", for example? And execute "SELECT username FROM all_users;" It'll show you whether you have or have not user "scott" available in your database.
Thanks for replying my post. I still have questions regarding SQL Plus worksheet. Could your guys tell me how can I determin the service name use by oracle database? Should I check the home directory to see where the database is installed or along the install process, I already chose the service name. if you guy have time, please help me out. Thanks!
Service name is the concatenation of instance_name/db_name and db_domain with a dot (.) in between. You can find it in TNSNAMES.ORA file (should be in your oracle_home\network\admin folder). In there you're supposed to have something like this: