The PayPal for WooCommerce plugin will allow you to enable Express Checkout with guest checkout forced, however, this does not always guarantee that you'll get the option for all buyers. Depending on the sender and receiver countries involved PayPal may simply not provide it. Are you using that plugin already, by chance? If so, and you think you have it all configured correctly, you might try disabling the option to "Use WordPress Locale Code" and see if that helps.
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