Set REMOTE_LOGIN_PASSWORDFILE=EXCLUSIVE|SHARED in init.ora

Create a new password file in %ORACLE_HOME%\database (Windows) or cd $ORACLE_HOME/dbs (linux) 
Name of password file must be pwd.ora (Windows) or orapw (Linux)

cd %ORACLE_HOME%\database
orapwd file=pwd%ORACLE_SID%.ora entries=5

If the password file already exists:
orapwd file=pwdSID.ora entries=5 FORCE=Y


Starting with Oracle Database 12.2.0.1 the orapwd utility – used to create Password Files for remote authentication – enforces complexity rules for the provided password.  If you cannot set a strong password, you can use the old 12c Release 1 format using the format parameter – the default for this parameter is 12.2.

orapwd file=$ORACLE_HOME/dbs/orapwDB01 password=oracle format=12 force=y

To verify the format of your Password File, just use the describe command of the orapwd utility.

orapwd describe file=$ORACLE_HOME/dbs/orapwC40DEVS
Password file Description : format=12.2

orapwd describe file=$ORACLE_HOME/dbs/orapwC39PRDS
Password file Description : format=12


Once the ora pwd file is created grant sysdba:

Grant SYSDBA/SYSOPER to users:
SQL> grant sysdba to sys;

Grant succeeded.

View pwd file and grants:
select * from V$PWFILE_USERS;

USERNAME                       SYSDB SYSOP SYSAS SYSBA SYSDG SYSKM     CON_ID
------------------------------ ----- ----- ----- ----- ----- ----- ----------
SYS                            TRUE  TRUE  FALSE FALSE FALSE FALSE          0