ORA-12505, TNS:listener does not currently know of SID given in connect descriptor

The database trying to be connected to is not running. Ensure the Oracle database is running and that the TNS entry you are using is correct.

Resolution

Start the Oracle Database

Windows start the OracleServiceXE service.

Linux login as oracle and run dbstart.

Test TNSTo test TNS run tnsping on the command line and give it the TNS name you are trying to use:

tnsping <tns name>