I have an error before I connect to the user Oracle 12c
Possible reasons:


You have SQLNET.AUTHENTICATION_SERVICES=(NONE) set in sqlnet.ora which disables operating system authentication based on group membership ($ORACLE_HOME/network/admin/sqlnet.ora, or $TNS_ADMIN/sqlnet.ora, it $TNS_ADMIN is set).
your user (oracle) is not the member of the group configured to be used for operating system authentication. Groups can be found in $ORACLE_HOME/rdbms/lib/config.c.
You have the environmental variable TWO_TASK set to something which is automatically added to sqlplus connection attemts as a TNS alias. For example if TWO_TASK is set to ORCL, sqlplus / as sysdba will become sqlplus /@ORCL as sysdba in the background, where operating system authentication does not work.