Before you open the server manager check your environment variables whether they had been set correctly.

ORACLE_HOME
LD_LIBRARY_PATH
PATH
CLASSPATH
ORACLE_SID=sid

To me it looks like that you have not set your demo_db's sid on the ORACLE_SID env variable. as a result, when you get to open the server manager and ask it to connect internal, it does not know of where to connect.

Also make sure that the database name is set on the tnsnames.ora and the listeners are there to listen.

Good luck
Sam