The system is very obtuse.

When you select an award level from the "use my points" screen, it takes you back to regular reservations but includes the PC code for that particular award.

If you book the applicable class and the applicable days (i.e., you're not trying to book an SUV on a regular award, or a Wednesday-Friday rental on a weekend award) and it still says the code is invalid, you've probably hit a blackout date. I'm not aware of any other way to determine blackout dates.

Another confusing area is that when you book it'll still show you having to pay considerable taxes and fees. But when you actually return the car, the taxes and fees are considerably less. So far I've only used points for Manhattan rentals and they've charged me exactly $0 when I went to return the car, even though both reservations had like $40-50 in taxes. You may still have to pay some fees at locations that have airport service fees or other regulatory fees that aren't calculated as a percentage of the cost.