Hello @skwish
Thank you for reaching out to us, we are here to help.
This issue has been reported many times before, and we are working on improving it, but let me explain what is causing this error. When you check out using our plugin, you will always have to press the yellow (by default) PayPal button. This button is placed on the page, by replacing the WooCommerce button via JavaScript. Sometimes this JavaScript does not function correctly and this causes the failure your customer has encountered. This failure can be caused by conflicting plugins, but in your case it was most likely caused by the customer using an older browser or OS when making the purchase.
We are still working on improving the browser compatibility, but for now we have added a better error description when this issue occurs (this will be included in the upcoming release 2.0.2). This will inform the customer to possibly try another browser or device to create the order. Later updates hopefully will improve the overall situation and avoid this error from occurring.
Let me know if this answers your question.
Kind regards,
Joost