This is not a customer support forum. It is a community support forum. We don't handle money issues here. You have to contact Playstation customer support by phone or live chat (https://support.us.playstation.com/app/contact_options). There are a number of possible causes, which are hard to determine as you didn't describe what happened when you tried to redeem the card. You could have read the code incorrectly from the card. The cashier at Walmart might have processed the card incorrectly at checkout. The payments system could be temporarily offline. You need to contact customer support and have them figure it out.