I am having the same issue and have been having it for a while now.
I have posted about it here: "Purchase Failed" in Online / Can't Change Anything on Cars - Answer HQ
and here: "Purchase Failed" Error / Unable to Customize Cars (Online) - Answer HQ (ea.com)
No help in either thread yet. In the second one, you can also see 7 other cases online that I've found where someone is having the same issue, with no suggested fix or technical help.
Starting to think I just won't be able to play the game again, which really sucks because I loved this game.
