I am having the same issue. I cannot change any parts on any of my cars, rendering the game pretty much pointless. I get "purchase failed" when selecting a part that I don't own, or if selecting a part that I do own, it simply reverts back to what it was after leaving the menu.
I made a thread about this but I guess it got merged with this one:
Purchase failed when buying cars - Answer HQ (ea.com)
In my post, you can see 7 other cases I have found online of people having the same issue.
Update on this: I just started the game to check and it is now working properly for the first time in months. When I started, it had some loading bars I hadn't seen before which said something about "synchronizing inventory" and such. I'm now able to equip parts on my cars. Thank you so much whoever fixed this.
