Hey, Legends!
As you might know, each multiplayer season focuses on certain cars. Every time a multiplayer series begins, the list of cars needed for that series is updated. Sometimes Platinum or Legend leagues don't have specific cars set, so you will only have access to the cars offered in the lower leagues. In this case, the icon to select cars will be grayed out for players in the Platinum or Legend league. This is intended! Don't worry, though; these cars will be usable in future MP seasons.