Forza Horizon 2 - How do I unlock/buy cars?

[Draeger]

Not me actually, asking this on behalf of our 7 year old son who loves playing this game. He tells me he can't buy any cars despite having a decent amount of credits and his level is apparently at 19 right now. He gets this message below. Any suggestions? I know next to nothing about how this all works as I don't play the game. Oh, it's on an Xbox One if that helps.

Thanks in advance for any advice. Screenshots below:-

 

[Draeger]

Should be via the Autoshow/garage at the festival hub. What method is he trying to use?

Cheers. When he goes to the festival hub autoshow, this is what he gets:-

 

[Draeger]

Thanks guys, will give all suggestions a go and see what happens.