There are two senses of "the highest good." In one sense, it refers to that which is always good no matter what and that which is required for all other goods. This is dutifulness. In the other sense, it refers to the best of goods, even if part of that state is only contingently good. The highest good looked at this way combines virtuousness with happiness.
The highest good is the object of pure practical reason, so we cannot utilize the latter unless we believe the former to be achievable. However, in this world, virtue does not necessarily lead to happiness or vice-versa. To aim at one is not to aim at the other, and it depends on chance whether the rest of the world will bridge the gap, rewarding the good. So it seems that pure practical reason cannot apply to us after all.
The flaw in this argument is that it assumes that we exist only phenomenally and thus can be rewarded only here in the phenomenal world. However, on the contrary, we can detect our noumenal existence as autonomous causes. Since we exist in a way other than as we detect ourselves here and now, there may be other times for us to be rewarded.
What happens when practical reason's maxims are connected to theoretical positions about which theoretical reason says nothing? Practical reason simply demanding the object of its desire is not an acceptable reason to believe. Just because the notion of mystic union with God, for example, happens to appeal to me is no reason for me to think that it will happen. But when it is pure practical reason that makes demands, that is a different matter. In that case, the demand is necessary for the faculty of reason as a whole and so commands assent.
The highest good requires the highest level of virtue. This, we can tell by looking inwardly, does not exist in us now, nor is it likely to exist in the foreseeable future. In fact, the only way the fallible human will can turn into the perfect holy will is for it to take an eternity to perfect itself. Therefore, we can postulate that we are immortal. If we fail to make this postulation, either we are led to soften the demands of morality to make them achievable here and now, or we are led to make the absurd demand on ourselves that we must achieve the holy will here and now.
The highest good requires the highest level of happiness as well, to reward the highest level of virtue. We cannot suppose this will come about by chance, even given an infinite amount of time. We need to make the supposition that there is an omniscient, omnipotent God who can order the world justly and reward us for virtue.