I'm Not Allowed to Purchase anything on Minecraft

Okay so to start off i was ip banned from buycraft for chargebacks anyway that server canceled so i charged back all my money i donated to it anyway so now i have to have my friends buy me ranks but right now there not up to it so i'm at school and i'm trying to buy the rank myself and it is saying ''you are not allowed to purchase this item'' and i have bought ranks at school before so i'm not sure what's the problem help?

ps. If you don't know what chargebacks are they are a plug in that minecraft servers use to take in donations to that server any hope you guys can help have a good day.

Could be the school network blocking you and before it may have glitched and allowed you to buy ranks or what ever