Como resolver a mensagem 'Status: Falha -Teste falhou: Erro IO: O Adaptador de Rede não conseguiu estabelecer a conexão' do Oracle 12

Aqui estão os seis passos que você pode tentar se conectar à sua base de dados a partir do desenvolvedor sql.
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1. Configure as variáveis ORACLE_HOME, ORACLE_SID, PATH >li>li>Try connecting to the database :
   >

• >>li>Navigate to ORACLE_HOME/bin>li>type the command - "sqlplus / as sysdba " to get into sqlplus>li> Se diz CONNECTED TO AN IDLE INSTANCE, digite o comando STARTUP; para iniciar a base de dados.
  >
• Se diz CONECTED TO so & so INSTANCE, então a base de dados está em funcionamento.

1. >li>Cheque se a base de dados está ouvindo.
   >

• • Navigate to ORACLE_HOME/bin
    >>/li>
  • digite o comando - "./lsnrctl status" e isto lhe dará a descrição de Listening Endpoint.
  • Se digitar o comando acima diz algo como "database is't listening". You can give the command "./lsnrctl start" - this would start the listener.

1. Check the endpoint description shown in the result of "./lsnrctl status"
   command and connect to the database by specifying the right SID, port number etc.
   
2. If you aren't sure of the username and password,
   

• • Get into sqlplus as sysdba (as mentioned in step 2)
  • Run the query - "select * from all_users;"
  • Confirm that the username you are trying to login with is present in the result of the above query.
  • Set a new password for the user with the command - "ALTER USER user_name IDENTIFIED BY new_password;"

1. Now connect to the database via sql developer with right credentials and Enjoy!!
   

PS: I haven't used Oracle 12, but I am very familiar with Oracle 11g. Assuming that there is no much difference in the way how we connect to the database, I have written this answer.