If the action is not able to be completed...Ex. Red Melee Attack Action when not engaged.

Does the color Red Shield modifier apply or is it also cancelled?

It still triggers because it is not an enhance modifier, nor us it a movement modifier. Only those types of modifiers will have no effect if the action is canceled. I think it's under "Cancel" in the rules reference. The list of what is a movement modifier and what is an enhance modifier can be found in the Learn to Play guide.

@Budgernaut I think you are correct except that Cancel is a specific game effect that nullifies an action, like when you spend a Stun or Immobilize token on an enemy. In this case the player can't choose any targets, since the unit is not engaged, so the Melee Attack action is ignored (this is not defined in the rules, I think because they assumed everyone would understand this) because it has no legal targets, not because the unit could not perform the action. In either case, 15.2 would apply to cancel movement and enhance modifiers of actions not performed the same as if it were cancelled.

I think we need a 15.3 added that states the corollary of 15.1 with something like "An action that can't be performed because it has no legal targets is cancelled."

I can't find anything that says you ignore your action if you can't complete it. I thought I had read it, but doing a search for "ignore," it doesn't show up.

Yeah, like I said, its not defined in RAW. It mentions 'ignored' many times in the rules, but never defines what that means (though it seems distinct from 'Cancelled', which specifically prevents a unit from taking an action or modifier it is otherwise capable of). The closest I can get to giving a precise rule is 10.2.i (and reconfirmed in 48) which says when targeting a melee attack you must choose a unit that you are engaged with. It never says anywhere to cancel or ignore or what to do with an effect without a target. It just seems like cancel is a very specific effect to nullify an action for a specific reason, not because an action can't be resolved or has no legal targets.