Im trying buy a theme with paypal , but it always say "Your payment can't be completed. Please return to the participating website and try again. " , I try many times and many payment methods like Bank Account , Credit card. none of them works.

My paypal account works OK, because I can buy order from other site.

this is the theme I'm trying to buy https://octobercms.com/theme/johnpajarillo-single-page-website

Try installing Hola and browse from the United States for example. Did the trick for us.