There are certain units that will show “Locked for Platoon”. This will only happen with defensive platoons, and only with select units. There are two different reasons this could be occurring.
The first reason is that the unit is a mega unit and it is trying to be used in a defensive platoon. No mega unit can be used in a defensive platoon.
You are viewing a unit specific platoon. As an example the Garage defenders can only use Light Vehicles.
Air units that are considered to have a “loiter time” (i.e. do not bomb and leave) can be used on defense. The only exception to this is the Ghostrider, for balance reasons.