linux – I have an error before connecting to the Oracle 12c user

After that, I connect normally.

I tried sqlplus as error sysdba is the same

Why this error occurs

I am a beginner

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

SQL * Plus: Launch 12.2.0.1.0 Production on Wednesday November 21 13:08:51 2018

Copyright (c) 1982, 2016, Oracle. All rights reserved.

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


Enter the username: system
Enter the password:
Last successful logon time: Wednesday, November 21, 2018 13:07:42 -05: 00

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