Ava here from Shopify.
We just had an unexpected issue that is causing some problems with the way our checkout is processing payments. Our developers are investigating this issue and hope to have it resolved soon. While I do not have an exact ETS for when this will be resolved, you can keep up to date here on our status page - https://status.shopify.com/. Payments will still go through if the customer enters their card details, so this error is only visual at the moment.
We should have this resolved soon, but do let me know if you are still seeing errors once it has been resolved!