The first thing I'd recommend doing is checking your abandoned checkouts, to see if the order attempt appears there. If it does, oftentimes an error message can be seen within the order, which should explain and clarify the issue further. You can follow the instructions below on how to check this on your end:
On your Shopify admin, go toOrders > Abandoned checkouts
Select your order, then scroll down to the order's History section and click on the line items to expand more information
Check for an error code in regards to the failed payment attempt
If you've located an error code but aren't sure of what it means, you can reach out to our live support team who can help clarify. We aren't able to do this via the Community Forums at this stage, but you can reach out to our support teamvia our Help Center here. Once logged in, select a topic that best fits your query or situation. From there, you'll be taken to a page with a list of relevant support articles, and you can scroll to the bottom of that page to select your preferred contact method with our team.
If you're currently testing your store's checkout, just a reminder that we have a couple of resources to help with placing a test order on your store:
Kimi | 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