By default HA deployments do not display or allow IPV6 addressing available for external traffic.
By default Virtual Address for cluster configuration is disabled when IPV6 addressing is not configured on the appliance(s).
To enable IPV6 based Virtual IP Addressing on HA devices.
1. Login to Secure Mobile Access Management Console
Go to System Configuration | Network Settings | Basic | Click 'Edit' | Select External Interface and enable IPV6 address for both Node 1 & Node 2:

2.How to get IPV6 address for appliance external interface.
ssh to the appliance and run ifconfig eth1 to get IPV6 address.

3. Enable External Interface to enter IPV6 address captured in Step 2.
NOTE: Perform the same on both nodes which would activate IPV6 Virtual address field.
NOTE: One Pending Changes being applied both nodes in HA would automatically synchronize. IPV6 is not supported on Internal interface.