You mean you can't join oneblock with /oneblock at all? Or when you try to type a command the game crashes? I suggest creating a bug report about it: https://www.mcfoxcraft.com/support/bug-report.1/select
Type /vote (ingame), click on 'Giftcards' and click on the giftcard that you want (if you have enough votepoints ofcourse). You will recieve a code and you can use this code to buy something in the store.
You don't have to worry about forgetting the code, you can see all the giftcard codes you have created by clicking on 'Your gift cards' in the /vote menu.