your purchase could not be completed .. I want a solution to this problem.

Contact Apple Support at:

iTunes Support - Use this form for assistance with account and billing, code redemption, or purchase questions - https://ssl.apple.com/emea/support/itunes/contact.html

or:

www.apple.com/support/itunes/ww/ --> https://support.apple.com/country-selector/itunes > select your country > Contact Support > Contact iTunes Store Support > Purchases, Billing & Redemption