voucher code

I bought my son 2 voucher cards ,one worked fine, the other was damaged, it got wet and when we scratched off the backing it came away in places and the letters were defaced. so we cant read it.  

Sooooo  can anyone tell me what to do? can we get a replacment? can we get a credit to the account?

Tesco wont refund ro replace, they said to come here..

Thx