TNS-12543: TNS:destination host unreachable I have a installed oracle 19c
but i am unable to login

Please refer the following screen shot



I am not unable to login
which user credential require to login
Have you looked at the alert log to verify that the database is running. Can you show your environment settings for ORACLE_SID, ORACLE_HOME, LD_LIBRARY_PATH, TNS_ADMIN and PATH? Do you have a listener running? Can you also show the contents of your listener.ora, sqlnet.ora and tnsnames.ora files? It could be that sqlplus can't find your database. Either using the listener or having ORACLE_SID set should get you to your database. You want to verify that the database and version of Sqlplus are both using the same Oracle Home.