In order to run an official series, members must license the car and track that is being raced for week. If you've just licensed and downloaded the car, but still have the Content Required button in the race panel, it means you still need the track that is being raced. Likewise if you have the track but not the car.
You may also see this if you are trying to get into a mixed class Race. Although you do not need to pay for ALL of the cars that are in that series, you do need to download, install and update the other car files so that you can see them when you are out on the track.