Whose name are you entering when you go to the "Donate" page? It should be exact in-game name of the person for whom you want to buy the rank. If you're entering that person's name and it gives you that message, it's likely that this player has a chargeback associated with their account (it may have been someone else donating for them) that needs to be cleared before they can buy or receive a rank. If that's the case, try asking that person to go to the "Donate" page themselves, and then to post here in Donation Help with their IGN if they get the same message.
Edit: If you keep getting this message no matter whose name you enter, an admin will need to take a look at it.
