If you unlocked all of the cars on another platform this is likely glitched. I solved it by unlinking my PSN account from my Epic account (on Epic's website), which allows you to start fresh and complete the new player challenges until you have 5 cars. In order to restore your account you will need to sign up for an Epic account with the new PSN account, unlink PSN from the account you just created, and then login to your old Epic account and link it to PSN again. As a warning this will mess up your preset cars.