If a customer is having trouble completing their purchase, and this appears to be an isolated incident (where you are not seeing this error replicated with other customers), this could indicate an issue with the customer's Laybuy account.
Customer account issues can be caused by a wide range of factors of which will likely not be able to be answered by anyone but the customer themselves. Due to this, it is best to advise your customer to contact our Customer Experience team at help@laybuy.com (Or advise them to submit a request here).