20% off Galaxy Store error

I used the galaxy store for the first time using 5 dollars off for your first purchase. I then went to make a second purchase for $54.99 and a 20% off coupon which is also up to 5 dollars off then popped up, when tapping redeem it said you already used this coupon which I had not actually done yet. I no longer have access to this 20% off coupon and halting my purchase until I can figure out how it can be used or a new one issued? Thank you.