This error is come up right after I finish all the recovery process, when I want to connect to the recovered database as sysdba. At that time I cannot even connect through RMAN or SQLPLUS, quite confusing at first. Luckily I have Metalink account and I found this workaround. Type this command to create new password file
ORAPWD FILE=/ORACLE/HOME/DBS/orapw<SID_NAME> PASSWORD=<PASSWD> ENTRIES=5
Password file format in unix is : orapw<sid>
Password file format in windows is : pwd<sid>.ora
Then edit your database PFILE by appending this command in the bottom of the file
REMOTE_LOGIN_PASSWORDFILE=EXCLUSIVE
Now you can try to connect again by typing
SQLPLUS / AS SYSDBA
GRANT SYSDBA TO SYS;
GRANT SYSOPER TO SYS;
You also need to check your SQLNET.ORA in $ORACLE_HOME/network/admin there should be this line
SQLNET.AUTHENTICATION_SERVICES=(NTS)
You also need to check if your current Oracle OS user is a member in oinstall and dba.
Hope this can help you 😀