Oracle 12c default sys password

I'm new to database and Oracle, but I need to learn it.

I just following this tutorial to install Oracle 12c, I almost on at the end of the steps.

I have finishing install the database with GUI interface, and now I want to start the database that I have installed with this following command:

sqlplus /nolog

and yea, I have entered to the database, and then I run this following command to connect as sys:
connect sys as sysdba

But it prompt Enter password:

Unfortunately, I don't know what is the default password for that.

I have try the password with CHANGE_ON_ INSTALL base on this following documentation, but it seems not work actually.

So, what is the default password..?, or any way to bypass that password and then change it if I forgot..?

EDIT: I following @Tejash—-ORA-00980 instructions to use this command to directly log into DB as a sysdba:

sqlplus / as sysdba

And the result is like this:

What username should I use..?, I have try it with sys and sysdba but it still prompt a Enter password:

EDIT 2: Here is my sqlnet.ora and tnsnames.ora files

and here is my $ORACLE_SID and $ORACLE_HOME values.