You need Security Plus license. You can check to see if you have it with a show version.
Licensed features for this platform:
Maximum Physical Interfaces : 8
VLANs : 20, DMZ Unrestricted
Inside Hosts : Unlimited
Failover : Active/Standby
VPN-DES : Enabled
VPN-3DES-AES : Enabled
VPN Peers : 25
WebVPN Peers : 2
Dual ISPs : Enabled
VLAN Trunk Ports : 8
Advanced Endpoint Assessment : Disabled
This platform has an ASA 5505 Security Plus license.
