After your payment, you might have clicked to go back through earlier pages in your browser. That will cause an error, and this screen will appear. But all is well!
Your payment probably processed just fine. This error will, however, stop the site from sending you the usual automatic email confirming your payment. In that case, if you need a receipt, please contact me here. <https://changemusic.jp/contact>
Your Credit Card company or PayPal will of course send their usual payment confirmation emails to you. Thanks again!