How do I fix my iPhone showing error as "Your purchase could not be completed" on App Store
[Re-Titled by Moderator]
A possible reason for seeing this message is Apple cannot find a functioning payment method to use in making this purchase. Note that even free apps may require the presence of a payment method, although it won't be charged if they are free. You may not have adequate balance, the kind of payment method required, or there may be problems with the method(s) you do have.
Use the document Add a payment method to your Apple Account - Apple Support to review your payment methods such as credit or debit cards. Problem methods will usually have a notice in red lettering below them. The article: If your payment method is declined in the App Store or iTunes Store - Apple Support has some information about declined payment methods.
For information about updating payment methods on various platforms (iPhone, Mac, etc.), click here --> If you need to change or update your Apple Account payment method - Apple Support
If there's an issue [such as you having a subscription or being a Family Sharing Organizer], add a new card first, then remove the old card.
If using Apple Account balance to pay for an item, ensure there is adequate balance to pay for the item, including tax if applicable. Read this article about how to check your Apple Account balance.--> Check your Apple Account balance - Apple Support Some purchases may require the presence of another payment method such as a credit card even if the final payment will be made by account balance.
If the information above does not track down the nature of the problem then you may find the fastest route to resolution would be to accept the invitation to contact Apple for assistance with this.
To contact billing Support about updating a payment method, do this:
1. Click on this link --> https://getsupport.apple.com/?PGF=PGF63006&category_id=SC0060&symptom_id=23372
2. Scroll down to the bottom.
3. Select a chat session or a phone call.
If that fails to work, this support article has various country telephone numbers for contacting Apple for support and service ➔ Contact Apple for support and service - Apple Support