Solution ORA-01031, ORA-01994

Error
ORA-01031: insufficient privileges

Solution
Give sysdba role to sys
SQL> grant sysdba to sys; 

If you get this error ->
ORA-01994: GRANT failed: password file missing or disabled

Run following command;
$ orapwd file=$ORACLE_HOME/dbs/orapw$ORACLE_SID password=alper entries=2 force=y
-> entries; number of users can login as sysdba role
-> force; enable to overwrite current pwd file

Check remote_login parameter
SQL> show parameter remote_login_passwordfile;

If remote_login_passwordfile is not "EXCLUSIVE";
Change it permanently:
add this into init.ora file;
*.remote_login_passwordfile='EXCLUSIVE'

Check who has sysdba role in database lastly
SQL> select * from V$PWFILE_USERS;