I assume you are in Laos and your payment instrument is Laotian. WordPress.com uses PayPal as the payment processor. PayPal supports payment by major credit cards only from supported countries (https://www.paypal.com/worldwide). Right now, Laos is not on the PayPal supported countries, unfortunately.
Unless you have a credit card that’s non-Laotian registered/issued, then there’s no way around it.
I’m in Hong Kong and, even here, payment to PayPal with Hong Kong credit cards is sometimes a bit icky.