First, tell me how many credits you redeemed, what type of Builder's Club you are trying to buy and how long (monthly, 6 month, etc.), and what error message, if any, you are seeing when you try to buy it.
I'll help you as soon as you reply.
Once you redeem a ROBLOX Card, you need to go to the Builders Club page and click on the ROBLOX Card button (not the Builders Club buttons). That shows you your balance and what you can buy with it.