How to resolve 'Your Purchase Could Not Be Completed' error on Apple Account?

Hello, I have been encountering the error YOUR PURCHASE COULD NOT BE COMPLETED, for a month now and every time I spoke to support they said wait 48 hours and after 48 hours I have made one or two transactions and the same error appeared again and again the same repeated words and waiting for 48 hours later today after the fifth time when I waited for 48 hours the same error appeared again.

[Re-Titled by Moderator]

Original Title: Purchase