While making the payment, I applied for the discount code but my transaction was unsuccessful. When I try again, it tells me the code is already used. What do I do?

That sounds upsetting. Don't lose the spirit. If the transaction failed you can reuse the voucher starting with 7 after 30 minutes as it gets blocked for safety measure. In case it is a voucher starting with 8 voila! reuse it right away. Please stand by till the mentioned time. If the issue persist we're all ears to guide you.