This all depends on a number of variables. The three most common being where your account is based, where the buyer account is based and what product you currently have integrated.
Your account: Depending on the country your account is based out of, there may be regulatory restrictions that prevent you from receiving funds not associated with a PayPal account.
Buyer account: Depending on the country/locale of the buyer, there may be regulatory restrictions that prevent them from paying via credit/debit card without first creating a PayPal account and linking a confirmed credit/debit card.
Product: By default, one of our base checkout products is Express Checkout, which does not offer a guest checkout option. Other products may be available to you depending on the origin of your account. Most notably our free product, Website Payments Standard.