Oracle tablespace encryption

Hi All,

I am trying to encrypt an existing tablespace and I though you could do it using:

SQL> alter database datafile 'D:\ORACLE\ORADATA\HZNTEST\TEST_TABLESPACE.DBF' encrypt;

alter database datafile 'D:\ORACLE\ORADATA\HZNTEST\TEST_TABLESPACE.DBF' encrypt

                                                                        *

ERROR at line 1:

ORA-01916: keyword ONLINE, OFFLINE, RESIZE, AUTOEXTEND or END/DROP expected

I am using Oracle 12c R1.

I have tried researching this and specifically found the following which seems to suggest ALTER DATABASE DATAFILE data_file ENCRYPT is supported in 12.1.

https://docs.oracle.com/database/122/ASOAG/configuring-transparent-data-encryption.htm#ASOAG620

Thanks

Hakan