There is no major issue on the eBay site to receive payments, so the issue has to lie with your buyer.
Do they have a valid card on their eBay account ? Do they have any unpaid item strikes, so blocked from buying from you? Are they not in the UK a new member with a limited number / amount for items they can purchase?
Many things can prevent them from paying you, but it stems from their account.
@claire4793cuba