Don't give discounts if u not going to keep ur promise that not how u make new clients

I was awarded a gift card which expires and a discount for my first purchase on UberEATS but when I try to utilise it the deboniors rosettenville declines my discounts because I only have to pay an amount of less the R10 after the order ,why give the discounts if I cannot utilise it what a **** so know i will lose out on the voucher because debonairs rosettenville rejecting my order