Can only send 1 unit for Combat Deployment
err... I'm quite sure of what happened, but it seems the forced tutorial-ish "VIP Guard Duty" deployment actually still going on, and never finished even after playing for roughly 7 hours... plus said "VIP Guard Duty" deployment didn't appear among the deployment list.
A friend just told me that I can abort ALL ongoing deployments by using square button while on deployment list screen and viola... the unit which stuck in that forced tutorial can now be used again...
weird... case closed I guess...