You must enable port-mirroring on your switch. If you attempt to apply a policy that requires port-mirroring, the mirror action will be disabled until you enable the port-mirroring.
On ExtremeSwitching X450-G2 and X460-G2 series switches, mirroring can be configured on the same port as egress ACLs. Mirroring can send packets to port x and you can install your rule at egress port x, and the rule should match your mirrored traffic.
Starting with ExtremeXOS 30.1, a single ACL can have up to four mirror actions (each with a different mirror instance) to accomplish mirroring the same packet to multiple destinations (port, ERSPAN/remote-ip, RSPAN/remote-tag, etc). This capability is not supported on ExtremeSwitching X620 and X440-G2 series switches.