Database should be in flashback mode. To achieve this:

SQL> ALTER SYSTEM SET db_recovery_file_dest_size=50G scope=both;
SQL> ALTER SYSTEM SET db_recovery_file_dest='/db/Db_1/fra';

SQL> select name,flashback_on from v$tablespace where flashback_on='NO';

NAME                           FLA
------------------------------ ---
TBS_INDEX_BIG                  NO
TBS_LOG                        NO

SQL> alter tablespace TBS_INDEX_BIG  flasjback ON;
Tablespace altered

SQL> select flashback_on from v$database;

FLASHBACK_ON
------------------
NO

SQL> alter database flashback on;

Database altered.

SQL> select * from v$recovery_file_dest;

NAME
--------------------------------------------------------------------------------
SPACE_LIMIT SPACE_USED SPACE_RECLAIMABLE NUMBER_OF_FILES
----------- ---------- ----------------- ---------------
/db/fra1 5.3687E+10   63750144          31875072               2

SQL> alter database convert to snapshot standby;

SQL> alter database open;

Database altered.

SQL> select flashback_on from v$database;

FLASHBACK_ON
------------------
RESTORE POINT ONLY

SQL> select db_unique_name, open_mode, database_role from v$database;

DB_UNIQUE_NAME                 OPEN_MODE            DATABASE_ROLE
------------------------------ -------------------- ----------------
RDB4                           READ WRITE           SNAPSHOT STANDBY

SQL> select name, guarantee_flashback_database from v$restore_point;

NAME
--------------------------------------------------------------------------------
GUA
---
SNAPSHOT_STANDBY_REQUIRED_06/28/2016 19:15:49
YES

SQL> shutdown immediate;
SQL> startupmount;

SQL> ALTER DATABASE CONVERT TO PHYSICAL STANDBY;