I have an error before I connect to the user Oracle 12c After that, I log in normally.

I tried it sqlplus as sysdba error is the same

Why this error occurs

I'm a beginner

[oracle@localhost ~]$ sqlplus "/as sysdba"

SQL*Plus: Release 12.2.0.1.0 Production on Wed Nov 21 13:08:51 2018

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ERROR:
ORA-01017: invalid username/password; logon denied


Enter user-name: system
Enter password: 
Last Successful login time: Wed Nov 21 2018 13:07:42 -05:00

Connected to:
Oracle Database 12c Enterprise Edition Release 12.2.0.1.0 - 64bit Production
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.