Ted here from Shopify, thanks for posting your query.
I'm just going to ask a few questions so I can gather a bit more context:
Who is your payment gateway provider? Which gateway did the customer try to use?
Have they mentioned the exact error they are seeing or sent you a screenshot of the error? If so, could you share it with me?
Could you double-check that your gateway is not in test mode? For example, if you are using Shopify Payments, please disable Test Mode as shown in this guide.
After that, can you place an order using Test mode as shown in this guide?
Do you know what type of payment method they used? For example, was it a credit, debit or a pre-load card?
Ted | Social Care @ Shopify - Was my reply helpful? Click Like to let me know! - Was your question answered? Mark it as an Accepted Solution - To learn more visit the Shopify Help Center or the Shopify Blog