I have installed oracle 12c on centos 8. sqlplus>sys / as sysdba>startup, it starts the ORCL database. If I write, alter user HR account unlock; it says, user does not exist or something like that. During installation I did not set password for HR or I did not get to the window to set password for HR user. Now how can I get HR user and set password and how can I use this HR user now?