Encryption & Decryption

Database Security, we are going to use AES 256 Symmetric Encryption. We will be using RSA for Asymmetric Key Encryption, 1024 Bits.

We got the code working for the seond case but for the first, WHEN:

CREATE SYMMETRIC KEY sym_Key WITH ALGORITHM =
       AES_256 ENCRYPTION BY ASYMMETRIC KEY asym_Key
GO

THEN:

-- Msg 15314, Level 16, State 1, Line 1
-- Either no algorithm has been specified or the bitlength and the algorithm specified for the key are not available in this installation of Windows.

What can be the way out to be able to create the AES 256 Symmetric key.