Can't Buy Skin for Champion I Own

I recently bought darius after playing him in one for all. As I'm writing this it's my birthday and i got a ton RP so I want to buy Dunkmaster Darius but it states that it's not implemented. I don't know if this is cause I'm on the new client or something else, either way, please help me! (I also can't play game modes on the new client so also fix this)