The error message usually reads
"The Coupon code isn't valid. Verify the code and try again"
This is a fault which we are aware of and is caused by a customer using the code and then going backwards in the checkout process to amend their address or simply to continue shopping. On the customers' return the code no longer works. The system mistakenly believes that the code has been used. When it hasn't.
Please contact us and tell us the issue and we will refund your voucher and return your points so that you can create a fresh voucher code.