Correct, you can't use them with Award or Miles + Cash tickets (which is essentially considered to be an Award ticket where you have partially purchased the miles). Most here don't consider Miles + Cash to be a good deal unless you don't have enough miles as you are paying for miles at the rate of 1.9 cents per mile (most people value miles at less than that). This is the cheaper than straight miles purchases (3.5 cents per mile), but miles purchases have more flexibility in the increments of miles that can be purchased than Miles + Cash awards.