Hyde here from Shopify.
That's strange, though hard to diagnose without being able to see it. Could you provide a screenshot of the error message and a link to your store, please?
Also, the more context you could provide the better I can help you! So could you tell me how long you've been open for, whether you've set your Paypal account up properly following our help guide, and whether this happens with every customer attempting to purchase by Paypal on your store?
I look forward to hearing from you.
All the best, Hyde.