From the configuration guide (emphasis in mine):
"You can use any unused interface on the device as the failover link; however, you cannot specify an interface that is currently configured with a name. The failover link interface is not configured as a normal networking interface; it exists for failover communication only. This interface should only be used for the failover link (and optionally for the Stateful Failover link)."
Using an Etherchannel for failover should work (reference Failover Guidelines for Etherchannels) but subinterfaces no. I would say the parent interface that your attempted failover subinterface belongs to (along with the other active subinterfaces you mentioned) would make it a used interface.
