Payment field shows "This field is required"?

A user sent me a message that he tried to place his order on our form, but got an error message showing that the expiration date field was required, but he had selected an expiration date.  Why is it doing this?

Unfortunately, it would be hard for us to investigate if we do not have enough details, how is he/she filling out the form? If the total items that the payment field is "0" and he/she won't be able to proceed especially if he/she has filled out the credit card details. The one you shared is a general warning message. 

I cloned your form and here's how I can trigger that message:

It says "This field is required", it is not specific on the expiration date, but instead specific on the whole payment field. Your payment field is set to "Required", your user might not have selected a product to purchase.

If you could get more information on the information, that would help us better. Please do ask your customer to fill out everything and make sure that there's a product being selected. 

We'll wait for your response.