With PayPal Express, the default is set to ask customers to log in to PayPal (or create a PayPal Account). You can enable guest payments to let customers pay with their credit card even if they don't have their own PayPal account. To enable this feature, you need to have linked your own credit card to your PayPal business account.
To enable guest payments, you need to change the settings in your PayPal account settings. Learn more about enabling guest payments at PayPal.