To me it seems that he can't make a purchase from the website using his PayPal account, because he doesn't have a credit card linked to that account. Instead he has 'PayPal' credits, which he got via a bank transfer. I use PayPal all the time to buy plat, but I have my Visa card linked to it. I have no idea if it works or doesn't work if you 'only' have PayPal credits and no credit card linked to your PP account.