“Your purchase could not be completed”

Hello,

Did you make the $40 initial purchase? If not, you may have an account security compromise.

If you made the purchase but received the error erroneously this will help you request assistance.

See your purchase history for the App Store, iTunes Store, and more - Apple Support

"How to see your purchase history

1. Go to reportaproblem.apple.com.
2. Sign in with your Apple ID and password.
3. A list of your recent purchases appears. If you’re not sure what you were charged for but you know the amount of the charge, search for the amount. If you're looking for information about older purchases, see your purchase history on your device.

Learn what to do if you don’t see the item you’re looking for or if you see unexpected charges."

I you didn't, this is more relevant.

"If you see purchases that you don't recognize or unexpected charges

• If you see items in your purchase history that you don't remember buying, find out whether a family member bought the item. If family members have access to your device, you might want to require a password for every purchase. If someone else is using your Apple ID and password, change your Apple ID password.
• Learn what to do if you don't recognize a charge on your statement from your bank or financial institution.
• Learn how to request a refund."

I hope this helps.