Voucher cannot be used.

I received a voucher to use at KFC. I have tried to use the voucher on the app and online but , when the voucher is used the payment won't be processed. I tried to use the voucher in store and it says it's an error. I have e mailed kfc which asked for the details which I did forward. Up to now I have gotten. No feed back and I still can't use the voucher.