MACsec is not supported on the Nexus 9K 9300-FX switches with NX-OS version 7.x. Customer must upgrade to NX-OS version 9.2 or later to allow enabling the MACsec feature.
Configuration statement 'feature macsec' is not available in NX-OS version 7.x; this becomes available with 9.2 and later.
Cisco datasheet for the Nexus 9300-FX switches states MACsec is supported: "all the ports in the 9300-FX switches capable of greater than or equal to 10-Gbps, support the IEEE 802.1ae MAC Security (MACsec) standard on all downlink and uplink ports, allowing traffic encryption at the physical layer and providing secure server, border leaf, and leaf-to-spine connectivity"
Reference:
https://www.cisco.com/c/en/us/products/collateral/switches/nexus-9000-series-switches/datasheet-c78-738259.html
