Hello ChaiwatS.,
Thank you for using Apple Support Communities!
I understand from your post that you have received a message stating, "Your Purchase Could Not Be Completed.", when trying to make a purchase in the App Store. To assist you in resolving this type of issue I recommend reviewing the great information Johnathan UK provides here: Purchase cannot be completed | Official Apple Support Communities
Best Regards.
Thank you for using Apple Support Communities!
I understand from your post that you have received a message stating, "Your Purchase Could Not Be Completed.", when trying to make a purchase in the App Store. To assist you in resolving this type of issue I recommend reviewing the great information Johnathan UK provides here: Purchase cannot be completed | Official Apple Support Communities
Best Regards.