Wanted to know how to use the failover port ? That port is nt seen as interface to be assigned in the HA VLAN

When u went thru the setup did u select network failover? If so then in the failover section you can choose which interface to select for configsync, mirroring and failover.

See https://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/bigip-device-service-clustering-11-2-1/2.html