While startup standby database it fails with ORA-01154

Problem Description
Whenever you try to open or shutdown a standby database it fails with error ORA-01154
SQL> connect / as sysdba
Connected.

SQL> alter database open
ORA-01154: database busy. Open, close, mount, and dismount not allowed now

SQL> shutdown
ORA-01154: database busy. Open, close, mount, and dismount not allowed now

Cause of the Problem
The database is a standby database and it is in managed recovery mode. To verify the database is in managed recovery mode, enter this command

SQL> SELECT RECOVERY_MODE FROM V$ARCHIVE_DEST_STATUS;

If it returns MANAGED then the standby database is in managed recovery mode.

If database is in managed recover mode then it is not allowed to do open or shutdown the database.

Solution of the Problem
To take the database out of managed recovery mode, enter this command

SQL> ALTER DATABASE RECOVER MANAGED STANDBY DATABASE CANCEL;

You can now open the database as,
SQL> ALTER DATABASE OPEN READ ONLY;

Or, you may now shutdown the database.
SQL> shutdown