It sounds like you may be using a third-party payment gateway, is this correct?
Can you make sure that the credentials you're getting from the respective third-party gateways you're using have been entered correctly on your Shopify admin's payment settings? If you've checked that your credentials have been entered exactly right, you can get in touch with the gateway's support team directly. They're best equipped to support you further with this matter, as they'll be able to look into your account in more detail to help investigate this situation for you.
Once this issue has been resolved, it definitely pays to take a closer look at the checkout process on your store, to ensure that this process is as smooth and easy as possible for your customers. To further help optimize this, some tips that come to mind include allowing guest checkouts on your store, providing multiple payment options, and implementing accelerated checkouts. Feel free to take a look at our resources below, for more information on this:
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