@gmae06: It seems like either the tablet or the gift card are intended for use outside the UK. If you have purchased a gift card from a high street retailer in the UK, then this suggests that the tablet is the issue. Can you confirm the full model number of the tablet, located on the back of the device?
We're having the same issue with gift cards bought today from amazon and instore at tesco I'm wondering if there's a google play store issue as so many have complained on amazon too.
I've checked the devices and all set to uk, phones are contract mobiles which have been topped up previously with no issue.