Sell all cars except one, then get arrested by the police repeatedly until you run out of impound strikes.
The "Challenge Series" in Carbon is locked until you finish the first race in Career Mode. These challenges are how you unlock bonus cars (like the ’71 Dodge Challenger or the Koenigsegg CCX). A 0% save unlocks the Challenge Series menu from the start. nfs carbon 0 save game
To resolve a "0 save game" issue, verify if the problem is technical (permissions/compatibility) or data-related (corruption/loss). For immediate progress restoration, downloading a 100% completion save file is the fastest remedy, provided the file versions match. Sell all cars except one, then get arrested